Tracker.Computation

Tracker.Computation

A Computation object represents code that is repeatedly rerun in response to reactive data changes. Computations don't have return values; they just perform actions, such as rerendering a template on the screen. Computations are created using Tracker.autorun. Use stop to prevent further rerunning of a computation.

Each time a computation runs, it may access various reactive data sources that serve as inputs to the computation, which are called its dependencies. At some future time, one of these dependencies may trigger the computation to be rerun by invalidating it. When this happens, the dependencies are cleared, and the computation is scheduled to be rerun at flush time.

The current computation (Tracker.currentComputation) is the computation that is currently being run or rerun (computed), and the one that gains a dependency when a reactive data source is accessed. Data sources are responsible for tracking these dependencies using Tracker.Dependency objects.

Invalidating a computation sets its invalidated property to true and immediately calls all of the computation's onInvalidate callbacks. When a flush occurs, if the computation has been invalidated and not stopped, then the computation is rerun by setting the invalidated property to false and calling the original function that was passed to Tracker.autorun. A flush will occur when the current code finishes running, or sooner if Tracker.flush is called.

Stopping a computation invalidates it (if it is valid) for the purpose of calling callbacks, but ensures that it will never be rerun.

Example:

// if we're in a computation, then perform some clean-up
// when the current computation is invalidated (rerun or
// stopped)
if (Tracker.active) {
  Tracker.onInvalidate(function () {
    x.destroy();
    y.finalize();
  });
}

Stopping a computation is irreversible and guarantees that it will never be rerun. You can stop a computation at any time, including from the computation's own run function. Stopping a computation that is already stopped has no effect.

Stopping a computation causes its onInvalidate callbacks to run immediately if it is not currently invalidated, as well as its stop callbacks.

Nested computations are stopped automatically when their enclosing computation is rerun.

Invalidating a computation marks it to be rerun at flush time, at which point the computation becomes valid again. It is rare to invalidate a computation manually, because reactive data sources invalidate their calling computations when they change. Reactive data sources in turn perform this invalidation using one or more Tracker.Dependency objects.

Invalidating a computation immediately calls all onInvalidate callbacks registered on it. Invalidating a computation that is currently invalidated or is stopped has no effect. A computation can invalidate itself, but if it continues to do so indefinitely, the result will be an infinite loop.

onInvalidate registers a one-time callback that either fires immediately or as soon as the computation is next invalidated or stopped. It is used by reactive data sources to clean up resources or break dependencies when a computation is rerun or stopped.

To get a callback after a computation has been recomputed, you can call Tracker.afterFlush from onInvalidate.

This property is initially false. It is set to true by stop() and invalidate(). It is reset to false when the computation is recomputed at flush time.

This property is a convenience to support the common pattern where a computation has logic specific to the first run.

Tracker.Dependency

A Dependency represents an atomic unit of reactive data that a computation might depend on. Reactive data sources such as Session or Minimongo internally create different Dependency objects for different pieces of data, each of which may be depended on by multiple computations. When the data changes, the computations are invalidated.

Dependencies don't store data, they just track the set of computations to invalidate if something changes. Typically, a data value will be accompanied by a Dependency object that tracks the computations that depend on it, as in this example:

var weather = "sunny";
var weatherDep = new Tracker.Dependency;

var getWeather = function () {
  weatherDep.depend()
  return weather;
};

var setWeather = function (w) {
  weather = w;
  // (could add logic here to only call changed()
  // if the new value is different from the old)
  weatherDep.changed();
};

This example implements a weather data source with a simple getter and setter. The getter records that the current computation depends on the weatherDep dependency using depend(), while the setter signals the dependency to invalidate all dependent computations by calling changed().

The reason Dependencies do not store data themselves is that it can be useful to associate multiple Dependencies with the same piece of data. For example, one Dependency might represent the result of a database query, while another might represent just the number of documents in the result. A Dependency could represent whether the weather is sunny or not, or whether the temperature is above freezing. Session.equals is implemented this way for efficiency. When you call Session.equals("weather", "sunny"), the current computation is made to depend on an internal Dependency that does not change if the weather goes from, say, "rainy" to "cloudy".

Conceptually, the only two things a Dependency can do are gain a dependent and change.

A Dependency's dependent computations are always valid (they have invalidated === false). If a dependent is invalidated at any time, either by the Dependency itself or some other way, it is immediately removed.

See the Meteor Manual to learn how to create a reactive data source using Tracker.Dependency.

dep.depend() is used in reactive data source implementations to record the fact that dep is being accessed from the current computation.

For reactive data sources that create many internal Dependencies, this function is useful to determine whether a particular Dependency is still tracking any dependency relationships or if it can be cleaned up to save memory.

ReactiveVar

To use ReactiveVar, add the reactive-var package to your project by running in your terminal:

meteor add reactive-var

A ReactiveVar holds a single value that can be get and set, such that calling set will invalidate any Computations that called get, according to the usual contract for reactive data sources.

A ReactiveVar is similar to a Session variable, with a few differences:

• ReactiveVars don't have global names, like the "foo" in Session.get("foo"). Instead, they may be created and used locally, for example attached to a template instance, as in: this.foo.get().

• ReactiveVars are not automatically migrated across hot code pushes, whereas Session state is.

• ReactiveVars can hold any value, while Session variables are limited to JSON or EJSON.

An important property of ReactiveVars — which is sometimes a reason for using one — is that setting the value to the same value as before has no effect; it does not trigger any invalidations. So if one autorun sets a ReactiveVar, and another autorun gets the ReactiveVar, a re-run of the first autorun won't necessarily trigger the second. By default, only primitive values are compared this way, while calling set on an argument that is an object (not a primitive) always counts as a change. You can configure this behavior using the equalsFunc argument.

EJSON

EJSON is an extension of JSON to support more types. It supports all JSON-safe types, as well as:

• Date (JavaScript Date)
• Binary (JavaScript Uint8Array or the result of EJSON.newBinary)
• User-defined types (see EJSON.addType. For example, Mongo.ObjectID is implemented this way.)

All EJSON serializations are also valid JSON. For example an object with a date and a binary buffer would be serialized in EJSON as:

{
  "d": {"$date": 1358205756553},
  "b": {"$binary": "c3VyZS4="}
}

Meteor supports all built-in EJSON data types in publishers, method arguments and results, Mongo databases, and Session variables.

Buffers of binary data are represented by Uint8Array instances on JavaScript platforms that support them. On implementations of JavaScript that do not support Uint8Array, binary data buffers are represented by standard arrays containing numbers ranging from 0 to 255, and the $Uint8ArrayPolyfill key set to true.

When you add a type to EJSON, Meteor will be able to use that type in:

• publishing objects of your type if you pass them to publish handlers.
• allowing your type in the return values or arguments to methods.
• storing your type client-side in Minimongo.
• allowing your type in Session variables.

Instances of your type must implement typeName and toJSONValue methods, and may implement clone and equals methods if the default implementations are not sufficient.

For example, the toJSONValue method for Mongo.ObjectID could be:

function () {
  return this.toHexString();
};

If your type does not have a clone method, EJSON.clone will use toJSONValue and the factory instead.

The equals method should define an equivalence relation. It should have the following properties:

• Reflexivity - for any instance a: a.equals(a) must be true.
• Symmetry - for any two instances a and b: a.equals(b) if and only if b.equals(a).
• Transitivity - for any three instances a, b, and c: a.equals(b) and b.equals(c) implies a.equals(c).

If your type does not have an equals method, EJSON.equals will compare the result of calling toJSONValue instead.

HTTP

HTTP provides an HTTP request API on the client and server. To use these functions, add the HTTP package to your project by running in your terminal:

meteor add http

This function initiates an HTTP request to a remote server.

On the server, this function can be run either synchronously or asynchronously. If the callback is omitted, it runs synchronously and the results are returned once the request completes successfully. If the request was not successful, an error is thrown. This is useful when making server-to-server HTTP API calls from within Meteor methods, as the method can succeed or fail based on the results of the synchronous HTTP call. In this case, consider using this.unblock() to allow other methods on the same connection to run in the mean time. On the client, this function must be used asynchronously by passing a callback.

Both HTTP and HTTPS protocols are supported. The url argument must be an absolute URL including protocol and host name on the server, but may be relative to the current host on the client. The query option replaces the query string of url. Parameters specified in params that are put in the URL are appended to any query string. For example, with a url of "/path?query" and params of {foo:"bar"}, the final URL will be "/path?query&foo=bar".

The params are put in the URL or the request body, depending on the type of request. In the case of request with no bodies, like GET and HEAD, the parameters will always go in the URL. For a POST or other type of request, the parameters will be encoded into the body with a standard x-www-form-urlencoded content type, unless the content or data option is used to specify a body, in which case the parameters will be appended to the URL instead.

When run in asynchronous mode, the callback receives two arguments, error and result. The error argument will contain an Error if the request fails in any way, including a network error, time-out, or an HTTP status code in the 400 or 500 range. In case of a 4xx/5xx HTTP status code, the response property on error matches the contents of the result object. When run in synchronous mode, either result is returned from the function, or error is thrown.

Contents of the result object:

statusCode Number

Numeric HTTP result status code, or null on error.

content String

The body of the HTTP response as a string.

data Object or null

If the response headers indicate JSON content, this contains the body of the document parsed as a JSON object.

headers Object

A dictionary of HTTP headers from the response.

Example server method:

Meteor.methods({checkTwitter: function (userId) {
  check(userId, String);
  this.unblock();
  try {
    var result = HTTP.call("GET", "http://api.twitter.com/xyz",
                           {params: {user: userId}});
    return true;
  } catch (e) {
    // Got a network error, time-out or HTTP error in the 400 or 500 range.
    return false;
  }
}});

Example asynchronous HTTP call:

HTTP.call("POST", "http://api.twitter.com/xyz",
          {data: {some: "json", stuff: 1}},
          function (error, result) {
            if (!error) {
              Session.set("twizzled", true);
            }
          });

Email

The email package allows sending email from a Meteor app. To use it, add the package to your project by running in your terminal:

meteor add email

The server reads from the MAIL_URL environment variable to determine how to send mail. Currently, Meteor supports sending mail over SMTP; the MAIL_URL environment variable should be of the form smtp://USERNAME:PASSWORD@HOST:PORT/.

If MAIL_URL is not set, Email.send outputs the message to standard output instead.

You must provide the from option and at least one of to, cc, and bcc; all other options are optional.

Email.send only works on the server. Here is an example of how a client could use a server method call to send an email. (In an actual application, you'd need to be careful to limit the emails that a client could send, to prevent your server from being used as a relay by spammers.)

// In your server code: define a method that the client can call
Meteor.methods({
  sendEmail: function (to, from, subject, text) {
    check([to, from, subject, text], [String]);

    // Let other method calls from the same client start running,
    // without waiting for the email sending to complete.
    this.unblock();

    Email.send({
      to: to,
      from: from,
      subject: subject,
      text: text
    });
  }
});

// In your client code: asynchronously send an email
Meteor.call('sendEmail',
            'alice@example.com',
            'bob@example.com',
            'Hello from Meteor!',
            'This is a test of Email.send.');

Assets

Assets allows server code in a Meteor application to access static server assets, which are located in the private subdirectory of an application's tree. Assets are not processed as source files and are copied directly into your application's bundle.

Static server assets are included by placing them in the application's private subdirectory. For example, if an application's private subdirectory includes a directory called nested with a file called data.txt inside it, then server code can read data.txt by running:

var data = Assets.getText('nested/data.txt');

Note: Packages can only access their own assets. If you need to read the assets of a different package, or of the enclosing app, you need to get a reference to that package's Assets object.

Package.js

A package is a directory containing a package.js file, which contains roughly three major sections: a basic description, a package definition, and a test definition. By default, the directory name is the name of the package.

The package.js file below is an example of how to use the packaging API. The rest of this section will explain the specific API commands in greater detail.

/* Information about this package */
Package.describe({
  // Short two-sentence summary.
  summary: "What this does",
  // Version number.
  version: "1.0.0",
  // Optional.  Default is package directory name.
  name: "username:package-name",
  // Optional github URL to your source repository.
  git: "https://github.com/something/something.git",
});

/* This defines your actual package */
Package.onUse(function (api) {
  // If no version is specified for an 'api.use' dependency, use the
  // one defined in Meteor 0.9.0.
  api.versionsFrom('0.9.0');
  // Use Underscore package, but only on the server.
  // Version not specified, so it will be as of Meteor 0.9.0.
  api.use('underscore', 'server');
  // Use iron:router package, version 1.0.0 or newer.
  api.use('iron:router@1.0.0');
  // Give users of this package access to the Templating package.
  api.imply('templating')
  // Export the object 'Email' to packages or apps that use this package.
  api.export('Email', 'server');
  // Specify the source code for the package.
  api.addFiles('email.js', 'server');
});

/* This defines the tests for the package */
Package.onTest(function (api) {
  // Sets up a dependency on this package
  api.use('username:package-name');
  // Allows you to use the 'tinytest' framework
  api.use('tinytest@1.0.0');
  // Specify the source code for the package tests
  api.addFiles('email_tests.js', 'server');
});

/* This lets you use npm packages in your package*/
Npm.depends({
  simplesmtp: "0.3.10",
  "stream-buffers": "0.2.5"});

Build plugins are created with Package.registerBuildPlugin. See the coffeescript package for an example. Build plugins are fully-fledged Meteor programs in their own right and have their own namespace, package dependencies, source files and npm requirements.

Package Description

Provide basic package information with Package.describe(options). To publish a package, you must define summary and version.

Package Definition

Define dependencies and expose package methods with the Package.onUse handler. This section lets you define what packages your package depends on, what packages are implied by your package, and what object your package is exported to.

Unit Tests

Set up your tests with the Package.onTest handler, which has an interface that's parallel to that of the onUse handler. The tests will need to depend on the package that you have just created. For example, if your package is the email package, you have to call api.use('email') in order to test the package.

If you used meteor create to set up your package, Meteor will create the required scaffolding in package.js, and you'll only need to add unit test code in the _test.js file that was created.

External Packages and Plugins

Meteor packages can include NPM packages and Cordova plugins by using Npm.depends and Cordova.depends in the package.js file.

Mobile Config File

If your Meteor application targets mobile platforms such as iOS or Android, you can configure your app's metadata and build process in a special top-level file called mobile-config.js which is not included in your application and is used only for this configuration.

The code snippet below is an example mobile-config.js file. The rest of this section will explain the specific API commands in greater detail.

// This section sets up some basic app metadata,
// the entire section is optional.
App.info({
  id: 'com.example.matt.uber',
  name: 'über',
  description: 'Get über power in one button click',
  author: 'Matt Development Group',
  email: 'contact@example.com',
  website: 'http://example.com'
});

// Set up resources such as icons and launch screens.
App.icons({
  'iphone': 'icons/icon-60.png',
  'iphone_2x': 'icons/icon-60@2x.png',
  // ... more screen sizes and platforms ...
});

App.launchScreens({
  'iphone': 'splash/Default~iphone.png',
  'iphone_2x': 'splash/Default@2x~iphone.png',
  // ... more screen sizes and platforms ...
});

// Set PhoneGap/Cordova preferences
App.setPreference('BackgroundColor', '0xff0000ff');
App.setPreference('HideKeyboardFormAccessoryBar', true);
App.setPreference('Orientation', 'default');
App.setPreference('Orientation', 'all', 'ios');

// Pass preferences for a particular PhoneGap/Cordova plugin
App.configurePlugin('com.phonegap.plugins.facebookconnect', {
  APP_ID: '1234567890',
  API_KEY: 'supersecretapikey'
});

Meteor supports a variety of add-on packages and third party libraries. While you can build great applications using only the Meteor core functionality, optional packages can make development even faster and better.

Packages can be added to a Meteor project with:

meteor add <package_name>

and removed with:

meteor remove <package_name>

Some of the packages that Meteor Development Group maintains include:

appcache

The appcache package stores the static parts of a Meteor application (the client side Javascript, HTML, CSS, and images) in the browser's application cache. To enable caching simply add the appcache package to your project.

• Once a user has visited a Meteor application for the first time and the application has been cached, on subsequent visits the web page loads faster because the browser can load the application out of the cache without contacting the server first.

• Hot code pushes are loaded by the browser in the background while the app continues to run. Once the new code has been fully loaded the browser is able to switch over to the new code quickly.

• The application cache allows the application to be loaded even when the browser doesn't have an Internet connection, and so enables using the app offline.

(Note however that the appcache package by itself doesn't make data available offline: in an application loaded offline, a Meteor Collection will appear to be empty in the client until the Internet becomes available and the browser is able to establish a DDP connection).

To turn AppCache off for specific browsers use:

Meteor.AppCache.config({
  chrome: false,
  firefox: false
});

The supported browsers that can be enabled or disabled include, but are not limited to, android, chrome, chromium, chromeMobileIOS, firefox, ie, mobileSafari and safari.

Browsers limit the amount of data they will put in the application cache, which can vary due to factors such as how much disk space is free. Unfortunately if your application goes over the limit rather than disabling the application cache altogether and running the application online, the browser will instead fail that particular update of the cache, leaving your users running old code.

Thus it's best to keep the size of the cache below 5MB. The appcache package will print a warning on the Meteor server console if the total size of the resources being cached is over 5MB.

If you have files too large to fit in the cache you can disable caching by URL prefix. For example,

Meteor.AppCache.config({onlineOnly: ['/online/']});

causes files in your public/online directory to not be cached, and so they will only be available online. You can then move your large files into that directory and refer to them at the new URL:

<img src="/online/bigimage.jpg">

If you'd prefer not to move your files, you can use the file names themselves as the URL prefix:

Meteor.AppCache.config({
  onlineOnly: [
    '/bigimage.jpg',
    '/largedata.json'
  ]
});

though keep in mind that since the exclusion is by prefix (this is a limitation of the application cache manifest), excluding /largedata.json will also exclude such URLs as /largedata.json.orig and /largedata.json/file1.

For more information about how Meteor interacts with the application cache, see the AppCache page in the Meteor wiki.

accounts-ui

A turn-key user interface for Meteor Accounts.

To add Accounts and a set of login controls to an application, add the accounts-ui package and at least one login provider package: accounts-password, accounts-facebook, accounts-github, accounts-google, accounts-twitter, or accounts-weibo.

Then simply add the {{> loginButtons}} helper to an HTML file. This will place a login widget on the page. If there is only one provider configured and it is an external service, this will add a login/logout button. If you use accounts-password or use multiple external login services, this will add a "Sign in" link which opens a dropdown menu with login options. If you plan to position the login dropdown in the right edge of the screen, use {{> loginButtons align="right"}} in order to get the dropdown to lay itself out without expanding off the edge of the screen.

To configure the behavior of {{> loginButtons}}, use Accounts.ui.config.

accounts-ui also includes modal popup dialogs to handle links from sendResetPasswordEmail, sendVerificationEmail, and sendEnrollmentEmail. These do not have to be manually placed in HTML: they are automatically activated when the URLs are loaded.

If you want to control the look and feel of your accounts system a little more, we recommend reading the useraccounts section of the Meteor Guide.

audit-argument-checks

This package causes Meteor to require that all arguments passed to methods and publish functions are checked. Any method that does not pass each one of its arguments to check will throw an error, which will be logged on the server and which will appear to the client as a 500 Internal server error. This is a simple way to help ensure that your app has complete check coverage.

Methods and publish functions that do not need to validate their arguments can simply run check(arguments, [Match.Any]) to satisfy the audit-argument-checks coverage checker.

coffeescript

CoffeeScript is a little language that compiles into JavaScript. It provides a simple syntax without lots of braces and parentheses. The code compiles one-to-one into the equivalent JS, and there is no interpretation at runtime.

CoffeeScript is supported on both the client and the server. Files ending with .coffee, .litcoffee, or .coffee.md are automatically compiled to JavaScript.

Namespacing and CoffeeScript

Here's how CoffeeScript works with Meteor's namespacing.

• Per the usual CoffeeScript convention, CoffeeScript variables are file-scoped by default (visible only in the .coffee file where they are defined.)

• When writing a package, CoffeeScript-defined variables can be exported like any other variable (see Writing Packages). Exporting a variable pulls it up to package scope, meaning that it will be visible to all of the code in your app or package (both .js and .coffee files).

• Package-scope variables declared in .js files are visible in any .coffee files in the same app or project.

• There is no way to make a package-scope variable from a .coffee file other than exporting it. We couldn't figure out a way to make this fit naturally inside the CoffeeScript language. If you want to use package-scope variables with CoffeeScript, one way is to make a short .js file that declares all of your package-scope variables. They can then be used and assigned to from .coffee files.

• If you want to share variables between .coffee files in the same package, and don't want to separately declare them in a .js file, we have an experimental feature that you may like. An object called share is visible in CoffeeScript code and is shared across all .coffee files in the same package. So, you can write share.foo for a value that is shared between all CoffeeScript code in a package, but doesn't escape that package.

Heavy CoffeeScript users, please let us know how this arrangement works for you, whether share is helpful for you, and anything else you'd like to see changed.

ecmascript

This package lets you use new JavaScript language features that are part of the ECMAScript 2015 specification but are not yet supported by all engines or browsers. Unsupported syntax is automatically translated into standard JavaScript that behaves the same way.

This video from the July 2015 Meteor Devshop gives an overview of how the package works, and what it provides.

Usage

The ecmascript package registers a compiler plugin that transpiles ECMAScript 2015+ to ECMAScript 5 (standard JS) in all .js files. By default, this package is pre-installed for all new apps and packages.

To add this package to an existing app, run the following command from your app directory:

meteor add ecmascript

To add the ecmascript package to an existing package, include the statement api.use('ecmascript'); in the Package.onUse callback in your package.js file:

Package.onUse(function (api) {
  api.use('ecmascript');
});

Supported ES2015 Features

Syntax

The ecmascript package uses Babel to compile ES2015 syntax to ES5 syntax. Many but not all ES2015 features can be simulated by Babel, and ecmascript enables most of the features supported by Babel.

Here is a list of the Babel transformers that are currently enabled:

• es3.propertyLiterals
  Makes it safe to use reserved keywords like catch as unquoted keys in object literals. For example, { catch: 123 } is translated to { "catch": 123 }.

• es3.memberExpressionLiterals
  Makes it safe to use reserved keywords as property names. For example, object.catch is translated to object["catch"].

• es6.arrowFunctions
  Provides a shorthand for function expressions. For example, [1, 2, 3].map(x => x + 1) evaluates to [2, 3, 4]. If this is used in the body of the arrow function, it will be automatically bound to the value of this in the enclosing scope.

• es6.literals
  Adds support for binary and octal numeric literals. For example, 0b111110111 === 503 and 0o767 === 503.

• es6.templateLiterals
  Enables multi-line strings delimited by backticks instead of quotation marks, with variable interpolation: js var name = "Ben"; var message = `My name is: ${name}`;

• es6.classes
  Enables class syntax:

  class Base {
  constructor(a, b) {
    this.value = a * b;
  }
  }</p>
  
  <p>class Derived extends Base {
  constructor(a, b) {
    super(a + 1, b + 1);
  }
  }</p>
  
  <p>var d = new Derived(2, 3);
  d.value; // 12
  

• es6.constants
  Allows defining block-scoped variables that are not allowed to be redefined:

  const GOLDEN_RATIO = (1 + Math.sqrt(5)) / 2;</p>
  
  <p>// This reassignment will be forbidden by the compiler:
  GOLDEN_RATIO = "new value";
  

• es6.blockScoping
  Enables the let and const keywords as alternatives to var. The key difference is that variables defined using let or const are visible only within the block where they are declared, rather than being visible anywhere in the enclosing function. For example:

  function example(condition) {
  let x = 0;
  if (condition) {
    let x = 1;
    console.log(x);
  } else {
    console.log(x);
    x = 2;
  }
  return x;
  }</p>
  
  <p>example(true); // logs 1, returns 0
  example(false); // logs 0, returns 2
  

• es6.properties.shorthand
  Allows omitting the value of an object literal property when the desired value is held by a variable that has the same name as the property key. For example, instead of writing { x: x, y: y, z: "asdf" } you can just write { x, y, z: "asdf" }. Methods can also be written without the : function property syntax: js var obj = { oldWay: function (a, b) { ... }, newWay(a, b) { ... } };

• es6.properties.computed
  Allows object literal properties with dynamically computed keys:

  var counter = 0;
  function getKeyName() {
  return "key" + counter++;
  }</p>
  
  <p>var obj = {
  [getKeyName()]: "zero",
  [getKeyName()]: "one",
  };</p>
  
  <p>obj.key0; // "zero"
  obj.key1; // "one"
  

• es6.parameters
  Default expressions for function parameters, evaluated whenever the parameter is undefined, ...rest parameters for capturing remaining arguments without using the arguments object:

  function add(a = 0, ...rest) {
  rest.forEach(n => a += n);
  return a;
  }</p>
  
  <p>add(); // 0
  add(1, 2, 3); // 6
  

• es6.spread
  Allows an array of arguments to be interpolated into a list of arguments to a function call, new expression, or array literal, without using Function.prototype.apply: js add(1, ...[2, 3, 4], 5); // 15 new Node("name", ...children); [1, ...[2, 3, 4], 5]; // [1, 2, 3, 4, 5]

• es6.forOf
  Provides an easy way to iterate over the elements of a collection: js let sum = 0; for (var x of [1, 2, 3]) { sum += x; } x; // 6

• es6.destructuring
  Destructuring is the technique of using an array or object pattern on the left-hand side of an assignment or declaration, in place of the usual variable or parameter, so that certain sub-properties of the value on the right-hand side will be bound to identifiers that appear within the pattern. Perhaps the simplest example is swapping two variables without using a temporary variable: js [a, b] = [b, a]; Extracting a specific property from an object: js let { username: name } = user; // is equivalent to let name = user.username; Instead of taking a single opaque options parameter, a function can use an object destructuring pattern to name the expected options:

  function run({ command, args, callback }) { ... }</p>
  
  <p>run({
  command: "git",
  args: ["status", "."],
  callback(error, status) { ... },
  unused: "whatever"
  });
  

• es7.objectRestSpread
  Supports catch-all ...rest properties in object literal declarations and assignments: js let { x, y, ...rest } = { x: 1, y: 2, a: 3, b: 4 }; x; // 1 y; // 2 rest; // { a: 3, b: 4 } Also enables ...spread properties in object literal expressions: js let n = { x, y, ...rest }; n; // { x: 1, y: 2, a: 3, b: 4 }

• es7.trailingFunctionCommas
  Allows the final parameter of a function to be followed by a comma, provided that parameter is not a ...rest parameter.

• flow
  Permits the use of Flow type annotations. These annotations are simply stripped from the code, so they have no effect on the code's behavior, but you can run the flow tool over your code to check the types if desired.

Polyfills

The ECMAScript 2015 standard library has grown to include new APIs and data structures, some of which can be implemented ("polyfilled") using JavaScript that runs in all engines and browsers today. Here are three new constructors that are guaranteed to be available when the ecmascript package is installed:

• Promise
  A Promise allows its owner to wait for a value that might not be available yet. See this tutorial for more details about the API and motivation. The Meteor Promise implementation is especially useful because it runs all callback functions in recycled Fibers, so you can use any Meteor API, including those that yield (e.g. HTTP.get, Meteor.call, or MongoCollection), and you never have to call Meteor.bindEnvironment.

• Map
  An associative key-value data structure where the keys can be any JavaScript value (not just strings). Lookup and insertion take constant time.

• Set
  A collection of unique JavaScript values of any type. Lookup and insertion take constant time.

• Symbol
  An implementation of the global Symbols namespace that enables a number of other ES2015 features, such as for-of loops and Symbol.iterator methods: [1,2,3][Symbol.iterator]().

• Polyfills for the following Object-related methods:

  • Object.assign
  • Object.is
  • Object.setPrototypeOf
  • Object.prototype.toString (fixes @@toStringTag support)
    

  Complete reference here.

• Polyfills for the following String-related methods:

  • String.fromCodePoint
  • String.raw
  • String.prototype.includes
  • String.prototype.startsWith
  • String.prototype.endsWith
  • String.prototype.repeat
  • String.prototype.codePointAt
  • String.prototype.trim
    

  Complete reference here.

• Polyfills for the following Array-related methods:

  • Array.from
  • Array.of
  • Array.prototype.copyWithin
  • Array.prototype.fill
  • Array.prototype.find
  • Array.prototype.findIndex

  Complete reference here.

• Polyfills for the following Function-related properties:

  • Function.prototype.name (fixes IE9+)
  • Function.prototype[Symbol.hasInstance] (fixes IE9+)

  Complete reference here.

jquery

jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development.

The jquery package adds the jQuery library to the client JavaScript bundle. It has no effect on the server.

In addition to the jquery package, Meteor provides several jQuery plugins as separate packages. These include:

• jquery-history
• jquery-layout
• jquery-waypoints

less

LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions. It allows for more compact stylesheets and helps reduce code duplication in CSS files.

With the less package installed, .less files in your application are automatically compiled to CSS and the results are included in the client CSS bundle.

markdown

This package lets you use Markdown in your templates. It's easy: just put your markdown inside {{#markdown}} ... {{/markdown}} tags. You can still use all of the usual Meteor template features inside a Markdown block, such as {{#each}}, and you still get reactivity.

Example:

{{#markdown}}I am using __markdown__.{{/markdown}}

outputs

<p>I am using <strong>markdown</strong>.</p>

modules

Though Meteor 1.2 introduced support for many new ECMAScript 2015 features, one of the most notable omissions was ES2015 import and export syntax. Meteor 1.3 fills that gap with a fully standards-compliant module system that works on both the client and the server, solves multiple long-standing problems for Meteor applications (such as controlling file load order), and yet maintains full backwards compatibility with existing Meteor code. This document explains the usage and key features of the new module system.

Enabling modules

We think you’re going to love the new module system, and that's why it will be installed by default for all new apps and packages. Nevertheless, the modules package is totally optional, and it will be up to you to add it to existing apps and/or packages.

For apps, this is as easy as meteor add modules, or (even better) meteor add ecmascript, since the ecmascript package implies the modules package.

For packages, you can enable modules by adding api.use("modules") to the Package.onUse or Package.onTest sections of your package.js file.

Now, you might be wondering what good the modules package is without the ecmascript package, since ecmascript enables import and export syntax. By itself, the modules package provides the CommonJS require and exports primitives that may be familiar if you’ve ever written Node code, and the ecmascript package simply compiles import and export statements to CommonJS. The require and export primitives also allow Node modules to run within Meteor application code without modification. Furthermore, keeping modules separate allows us to use require and exports in places where using ecmascript is tricky, such as the implementation of the ecmascript package itself.

While the modules package is useful by itself, we very much encourage using the ecmascript package (and thus import and export) instead of using require and exports directly. If you need convincing, here’s a presentation that explains the differences: http://benjamn.github.io/empirenode-2015

Basic syntax

Although there are a number of different variations of import and export syntax, this section describes the essential forms that everyone should know.

First, you can export any named declaration on the same line where it was declared:

// exporter.js
export var a = ...;
export let b = ...;
export const c = ...;
export function d() {...}
export function* e() {...}
export class F {...}

These declarations make the variables a, b, c (and so on) available not only within the scope of the exporter.js module, but also to other modules that import from exporter.js.

If you prefer, you can export variables by name, rather than prefixing their declarations with the export keyword:

// exporter.js
function g() {...}
let h = g();

// at the end of the file
export {g, h};

All of these exports are named, which means other modules can import them using those names:

// importer.js
import {a, c, F, h} from "./exporter";
new F(a, c).method(h);

If you’d rather use different names, you’ll be glad to know export and import statements can rename their arguments:

// exporter.js
export {g as x};
g(); // same as calling y() in importer.js

// importer.js
import {x as y} from "./exporter";
y(); // same as calling g() in exporter.js

As with CommonJS module.exports, it is possible to define a single default export:

// exporter.js
export default any.arbitrary(expression);

This default export may then be imported without curly braces, using any name the importing module chooses:

// importer.js
import Value from "./exporter";
// Value is identical to the exported expression

Unlike CommonJS module.exports, the use of default exports does not prevent the simultaneous use of named exports. Here is how you can combine them:

// importer.js
import Value, {a, F} from "./exporter";

In fact, the default export is conceptually just another named export whose name happens to be "default":

// importer.js
import {default as Value, a, F} from "./exporter";

These examples should get you started with import and export syntax. For further reading, here is a very detailed explanation by Axel Rauschmayer of every variation of import and export syntax.

Modular application structure

Before the release of Meteor 1.3, the only way to share values between files in an application was to assign them to global variables or communicate through shared variables like Session (variables which, while not technically global, sure do feel syntactically identical to global variables). With the introduction of modules, one module can refer precisely to the exports of any other specific module, so global variables are no longer necessary.

If you are familiar with modules in Node, you might expect modules not to be evaluated until the first time you import them. However, because earlier versions of Meteor evaluated all of your code when the application started, and we care about backwards compatibility, eager evaluation is still the default behavior.

If you would like a module to be evaluated lazily (in other words: on demand, the first time you import it, just like Node does it), then you should put that module in an imports/ directory (anywhere in your app, not just the root directory), and include that directory when you import the module: import {stuff} from "./imports/lazy". Note: files contained by node_modules/ directories will also be evaluated lazily (more on that below).

Lazy evaluation will very likely become the default behavior in a future version of Meteor, but if you want to embrace it as fully as possible in the meantime, we recommend putting all your modules inside either client/imports/ or server/imports/ directories, with just a single entry point for each architecture: client/main.js and server/main.js. The main.js files will be evaluated eagerly, giving your application a chance to import modules from the imports/ directories.

Modular package structure

If you are a package author, in addition to putting api.use("modules") or api.use("ecmascript") in the Package.onUse section of your package.js file, you can also use a new API called api.mainModule to specify the main entry point for your package:

Package.describe({
  name: "my-modular-package"
});

Npm.depends({
  moment: "2.10.6"
});

Package.onUse(function (api) {
  api.use("modules");
  api.mainModule("server.js", "server");
  api.mainModule("client.js", "client");
  api.export("Foo");
});

Now server.js and client.js can import other files from the package source directory, even if those files have not been added using the api.addFiles function.

When you use api.mainModule, the exports of the main module are exposed globally as Package["my-modular-package"], along with any symbols exported by api.export, and thus become available to any code that imports the package. In other words, the main module gets to decide what value of Foo will be exported by api.export, as well as providing other properties that can be explicitly imported from the package:

// In an application that uses my-modular-package:
import {Foo as ExplicitFoo, bar} from "meteor/my-modular-package";
console.log(Foo); // Auto-imported because of api.export.
console.log(ExplicitFoo); // Explicitly imported, but identical to Foo.
console.log(bar); // Exported by server.js or client.js, but not auto-imported.

Note that the import is from "meteor/my-modular-package", not from "my-modular-package". Meteor package identifier strings must include the prefix meteor/... to disambiguate them from npm packages.

Finally, since this package is using the new modules package, and the package Npm.depends on the "moment" npm package, modules within the package can import moment from "moment" on both the client and the server. This is great news, because previous versions of Meteor allowed npm imports only on the server, via Npm.require.

Local node_modules

Before Meteor 1.3, the contents of node_modules directories in Meteor application code were completely ignored. When you enable modules, those useless node_modules directories suddenly become infinitely more useful:

meteor create modular-app
cd modular-app
mkdir node_modules
npm install moment
echo 'import moment from "moment";' >> modular-app.js
echo 'console.log(moment().calendar());' >> modular-app.js
meteor

When you run this app, the moment library will be imported on both the client and the server, and both consoles will log output similar to: Today at 7:51 PM. Our hope is that the possibility of installing Node modules directly within an app will reduce the need for npm wrapper packages such as https://atmospherejs.com/momentjs/moment.

A version of the npm command comes bundled with every Meteor installation, and (as of Meteor 1.3) it's quite easy to use: meteor npm ... is synonymous with npm ..., so meteor npm install moment will work in the example above. (Likewise, if you don't have a version of node installed, or you want to be sure you're using the exact same version of node that Meteor uses, meteor node ... is a convenient shortcut.) That said, you can use any version of npm that you happen to have available. Meteor's module system only cares about the files installed by npm, not the details of how npm installs those files.

File load order

Before Meteor 1.3, the order in which application files were evaluated was dictated by a set of rules described in the Structuring Your Application section of the docs (see File Load Order subheading). These rules could become frustrating when one file depended on a variable defined by another file, particularly when the first file was evaluated after the second file.

Thanks to modules, any load-order dependency you might imagine can be resolved by adding an import statement. So if a.js loads before b.js because of their file names, but a.js needs something defined by b.js, then a.js can simply import that value from b.js:

// a.js
import {bThing} from "./b";
console.log(bThing, "in a.js");

// b.js
export var bThing = "a thing defined in b.js";
console.log(bThing, "in b.js");

Sometimes a module doesn’t actually need to import anything from another module, but you still want to be sure the other module gets evaluated first. In such situations, you can use an even simpler import syntax:

// c.js
import "./a";
console.log("in c.js");

No matter which of these modules is imported first, the order of the console.log calls will always be:

console.log(bThing, "in b.js");
console.log(bThing, "in a.js");
console.log("in c.js");

oauth-encryption

Encrypts sensitive login secrets stored in the database such as a login service's application secret key and users' access tokens.

Generating a Key

The encryption key is 16 bytes, encoded in base64.

To generate a key:

$ ~/.meteor/tools/latest/bin/node -e 'console.log(require("crypto").randomBytes(16).toString("base64"))'

Using oauth-encryption with accounts

On the server only, use the oauthSecretKey option to Accounts.config:

Accounts.config({oauthSecretKey: "onsqJ+1e4iGFlV0nhZYobg=="});

This call to Accounts.config should be made at load time (place at the top level of your source file), not called from inside of a Meteor.startup block.

To avoid storing the secret key in your application's source code, you can use Meteor.settings:

Accounts.config({oauthSecretKey: Meteor.settings.oauthSecretKey});

Migrating unencrypted user tokens

This example for Twitter shows how existing unencrypted user tokens can be encrypted. The query finds user documents which have a Twitter access token but not the algorithm field which is created when the token is encrypted. The relevant fields in the service data are then encrypted.

Meteor.users.find({ $and: [
    { 'services.twitter.accessToken': {$exists: true} },
    { 'services.twitter.accessToken.algorithm': {$exists: false} }
  ] }).
forEach(function (userDoc) {
  var set = {};
  _.each(['accessToken', 'accessTokenSecret', 'refreshToken'], function (field) {
    var plaintext = userDoc.services.twitter[field];
    if (!_.isString(plaintext))
      return;
    set['services.twitter.' + field] = OAuthEncryption.seal(
      userDoc.services.twitter[field],
      userDoc._id
    );
  });
  Meteor.users.update(userDoc._id, {$set: set});
});

Using oauth-encryption without accounts

If you're using the oauth packages directly instead of through the Meteor accounts packages, you can load the OAuth encryption key directly using OAuthEncryption.loadKey:

OAuthEncryption.loadKey("onsqJ+1e4iGFlV0nhZYobg==");

If you call retrieveCredential (such as Twitter.retrieveCredential) as part of your process, you'll find when using oauth-encryption that the sensitive service data fields will be encrypted.

You can decrypt them using OAuth.openSecrets:

var credentials = Twitter.retrieveCredential(token);
var serviceData = OAuth.openSecrets(credentials.serviceData);

Using oauth-encryption on Windows

This package depends on npm-node-aes-gcm, which requires you to have OpenSSL installed on your system to run. To install OpenSSL on Windows, use one of the binaries on this page. Don't forget to install the Visual Studio 2008 redistributables if you don't have them yet.

random

The random package provides several functions for generating random numbers. It uses a cryptographically strong pseudorandom number generator when possible, but falls back to a weaker random number generator when cryptographically strong randomness is not available (on older browsers or on servers that don't have enough entropy to seed the cryptographically strong generator).

Random.id([n])

Returns a unique identifier, such as "Jjwjg6gouWLXhMGKW", that is likely to be unique in the whole world. The optional argument n specifies the length of the identifier in characters and defaults to 17.

Random.secret([n])

Returns a random string of printable characters with 6 bits of entropy per character. The optional argument n specifies the length of the secret string and defaults to 43 characters, or 256 bits of entropy. Use Random.secret for security-critical secrets that are intended for machine, rather than human, consumption.

Random.fraction()

Returns a number between 0 and 1, like Math.random.

Random.choice(arrayOrString)

Returns a random element of the given array or string.

Random.hexString(n)

Returns a random string of n hexadecimal digits.

spacebars

Spacebars is a Meteor template language inspired by Handlebars. It shares some of the spirit and syntax of Handlebars, but it has been tailored to produce reactive Meteor templates when compiled.

Getting Started

A Spacebars template consists of HTML interspersed with template tags, which are delimited by {{ and }} (two curly braces).

<template name="myPage">
  <h1>{{pageTitle}}</h1>

  {{> nav}}

  {{#each posts}}
    <div class="post">
      <h3>{{title}}</h3>
      <div class="post-content">
        {{{content}}}
      </div>
    </div>
  {{/each}}
</template>

As illustrated by the above example, there are four major types of template tags:

• {{pageTitle}} - Double-braced template tags are used to insert a string of text. The text is automatically made safe. It may contain any characters (like <) and will never produce HTML tags.

• {{> nav}} - Inclusion template tags are used to insert another template by name.

• {{#each}} - Block template tags are notable for having a block of content. The block tags #if, #each, #with, and #unless are built in, and it is also possible define custom ones. Some block tags, like #each and #with, establish a new data context for evaluating their contents. In the above example, {{title}} and {{content}} most likely refer to properties of the current post (though they could also refer to template helpers).

• {{{content}}} - Triple-braced template tags are used to insert raw HTML. Be careful with these! It's your job to make sure the HTML is safe, either by generating it yourself or sanitizing it if it came from a user input.

Reactivity Model

Spacebars templates update reactively at a fine-grained level in response to changing data.

Each template tag's DOM is updated automatically when it evaluates to a new value, while avoiding unnecessary re-rendering as much as possible. For example, a double-braced tag replace its text node when its text value changes. An #if re-renders its contents only when the condition changes from truthy to falsy or vice versa.

Identifiers and Paths

A Spacebars identifier is either a JavaScript identifier name or any string enclosed in square brackets ([ and ]). There are also the special identifiers this (or equivalently, .) and ... Brackets are required to use one of the following as the first element of a path: else, this, true, false, and null. Brackets are not required around JavaScript keywords and reserved words like var and for.

A Spacebars path is a series of one or more identifiers separated by either . or /, such as foo, foo.bar, this.name, ../title, or foo.[0] (numeric indices must be enclosed in brackets).

Name Resolution

The first identifier in a path is resolved in one of two ways:

• Indexing the current data context. The identifier foo refers to the foo property of the current data context object.

• As a template helper. The identifier foo refers to a helper function (or constant value) that is accessible from the current template.

Template helpers take priority over properties of the data context.

If a path starts with .., then the enclosing data context is used instead of the current one. The enclosing data context might be the one outside the current #each, #with, or template inclusion.

Path Evaluation

When evaluating a path, identifiers after the first are used to index into the object so far, like JavaScript's .. However, an error is never thrown when trying to index into a non-object or an undefined value.

In addition, Spacebars will call functions for you, so {{foo.bar}} may be taken to mean foo().bar, foo.bar(), or foo().bar() as appropriate.

Helper Arguments

An argument to a helper can be any path or identifier, or a string, boolean, or number literal, or null.

Double-braced and triple-braced template tags take any number of positional and keyword arguments:

{{frob a b c verily=true}}

calls:

frob(a, b, c, Spacebars.kw({verily: true}))

Spacebars.kw constructs an object that is instanceof Spacebars.kw and whose .hash property is equal to its argument.

The helper's implementation can access the current data context as this.

Inclusion and Block Arguments

Inclusion tags ({{> foo}}) and block tags ({{#foo}}) take a single data argument, or no argument. Any other form of arguments will be interpreted as an object specification or a nested helper:

• Object specification: If there are only keyword arguments, as in {{#with x=1 y=2}} or {{> prettyBox color=red}}, the keyword arguments will be assembled into a data object with properties named after the keywords.

• Nested Helper: If there is a positional argument followed by other (positional or keyword arguments), the first argument is called on the others using the normal helper argument calling convention.

Template Tag Placement Limitations

Unlike purely string-based template systems, Spacebars is HTML-aware and designed to update the DOM automatically. As a result, you can't use a template tag to insert strings of HTML that don't stand on their own, such as a lone HTML start tag or end tag, or that can't be easily modified, such as the name of an HTML element.

There are three main locations in the HTML where template tags are allowed:

• At element level (i.e. anywhere an HTML tag could go)
• In an attribute value
• In a start tag in place of an attribute name/value pair

The behavior of a template tag is affected by where it is located in the HTML, and not all tags are allowed at all locations.

Double-braced Tags

A double-braced tag at element level or in an attribute value typically evalutes to a string. If it evalutes to something else, the value will be cast to a string, unless the value is null, undefined, or false, which results in nothing being displayed.

Values returned from helpers must be pure text, not HTML. (That is, strings should have <, not <.) Spacebars will perform any necessary escaping if a template is rendered to HTML.

SafeString

If a double-braced tag at element level evalutes to an object created with Spacebars.SafeString("<span>Some HTML</span>"), the HTML is inserted at the current location. The code that calls SafeString is asserting that this HTML is safe to insert.

In Attribute Values

A double-braced tag may be part of, or all of, an HTML attribute value:

<input type="checkbox" class="checky {{moreClasses}}" checked={{isChecked}}>

An attribute value that consists entirely of template tags that return null, undefined, or false is considered absent; otherwise, the attribute is considered present, even if its value is empty.

Dynamic Attributes

A double-braced tag can be used in an HTML start tag to specify an arbitrary set of attributes:

<div {{attrs}}>...</div>

<input type=checkbox {{isChecked}}>

The tag must evaluate to an object that serves as a dictionary of attribute name and value strings. For convenience, the value may also be a string or null. An empty string or null expands to {}. A non-empty string must be an attribute name, and expands to an attribute with an empty value; for example, "checked" expands to {checked: ""} (which, as far as HTML is concerned, means the checkbox is checked).

To summarize:

You can combine multiple dynamic attributes tags with other attributes:

<div id=foo class={{myClass}} {{attrs1}} {{attrs2}}>...</div>

Attributes from dynamic attribute tags are combined from left to right, after normal attributes, with later attribute values overwriting previous ones. Multiple values for the same attribute are not merged in any way, so if attrs1 specifies a value for the class attribute, it will overwrite {{myClass}}. As always, Spacebars takes care of recalculating the element's attributes if any of myClass, attrs1, or attrs2 changes reactively.

Triple-braced Tags

Triple-braced tags are used to insert raw HTML into a template:

<div class="snippet">
  {{{snippetBody}}}
</div>

The inserted HTML must consist of balanced HTML tags. You can't, for example, insert "</div><div>" to close an existing div and open a new one.

This template tag cannot be used in attributes or in an HTML start tag.

Inclusion Tags

An inclusion tag takes the form {{> templateName}} or {{> templateName dataObj}}. Other argument forms are syntactic sugar for constructing a data object (see Inclusion and Block Arguments).

An inclusion tag inserts an instantiation of the given template at the current location. If there is an argument, it becomes the data context, much as if the following code were used:

{{#with dataObj}}
  {{> templateName}}
{{/with}}

Instead of simply naming a template, an inclusion tag can also specify a path that evalutes to a template object, or to a function that returns a template object.

Note that the above two points interact in a way that can be surprising! If foo is a template helper function that returns another template, then {{>foo bar}} will first push bar onto the data context stack then call foo(), due to the way this line is expanded as shown above. You will need to use Template.parentData(1) to access the original context. This differs from regular helper calls like {{foo bar}}, in which bar is passed as a parameter rather than pushed onto the data context stack.

Function Returning a Template

If an inclusion tag resolves to a function, the function must return a template object or null. The function is reactively re-run, and if its return value changes, the template will be replaced.

Block Tags

Block tags invoke built-in directives or custom block helpers, passing a block of template content that may be instantiated once, more than once, or not at all by the directive or helper.

{{#block}}
  <p>Hello</p>
{{/block}}

Block tags may also specify "else" content, separated from the main content by the special template tag {{else}}.

A block tag's content must consist of HTML with balanced tags.

Block tags can be used inside attribute values:

<div class="{{#if done}}done{{else}}notdone{{/if}}">
  ...
</div>

If/Unless

An #if template tag renders either its main content or its "else" content, depending on the value of its data argument. Any falsy JavaScript value (including null, undefined, 0, "", and false) is considered false, as well as the empty array, while any other value is considered true.

{{#if something}}
  <p>It's true</p>
{{else}}
  <p>It's false</p>
{{/if}}

#unless is just #if with the condition inverted.

With

A #with template tag establishes a new data context object for its contents. The properties of the data context object are where Spacebars looks when resolving template tag names.

{{#with employee}}
  <div>Name: {{name}}</div>
  <div>Age: {{age}}</div>
{{/with}}

We can take advantage of the object specification form of a block tag to define an object with properties we name:

{{#with x=1 y=2}}
  {{{getHTMLForPoint this}}}
{{/with}}

If the argument to #with is falsy (by the same rules as for #if), the content is not rendered. An "else" block may be provided, which will be rendered instead.

If the argument to #with is a string or other non-object value, it may be promoted to a JavaScript wrapper object (also known as a boxed value) when passed to helpers, because JavaScript traditionally only allows an object for this. Use String(this) to get an unboxed string value or Number(this) to get an unboxed number value.

Each

An #each template tag takes a sequence argument and inserts its content for each item in the sequence, setting the data context to the value of that item:

<ul>
{{#each people}}
  <li>{{name}}</li>
{{/each}}
</ul>

The newer variant of #each doesn't change the data context but introduces a new variable that can be used in the body to refer to the current item:

<ul>
{{#each person in people}}
  <li>{{person.name}}</li>
{{/each}}
</ul>

The argument is typically a Meteor cursor (the result of collection.find(), for example), but it may also be a plain JavaScript array, null, or undefined.

An "else" section may be provided, which is used (with no new data context) if there are zero items in the sequence at any time.

You can use a special variable @index in the body of #each to get the 0-based index of the currently rendered value in the sequence.

Reactivity Model for Each

When the argument to #each changes, the DOM is always updated to reflect the new sequence, but it's sometimes significant exactly how that is achieved. When the argument is a Meteor live cursor, the #each has access to fine-grained updates to the sequence -- add, remove, move, and change callbacks -- and the items are all documents identified by unique ids. As long as the cursor itself remains constant (i.e. the query doesn't change), it is very easy to reason about how the DOM will be updated as the contents of the cursor change. The rendered content for each document persists as long as the document is in the cursor, and when documents are re-ordered, the DOM is re-ordered.

Things are more complicated if the argument to the #each reactively changes between different cursor objects, or between arrays of plain JavaScript objects that may not be identified clearly. The implementation of #each tries to be intelligent without doing too much expensive work. Specifically, it tries to identify items between the old and new array or cursor with the following strategy:

1. For objects with an _id field, use that field as the identification key
2. For objects with no _id field, use the array index as the identification key. In this case, appends are fast but prepends are slower.
3. For numbers or strings, use their value as the identification key.

In case of duplicate identification keys, all duplicates after the first are replaced with random ones. Using objects with unique _id fields is the way to get full control over the identity of rendered elements.

Let

The #let tag creates a new alias variable for a given expression. While it doesn't change the data context, it allows to refer to an expression (helper, data context, another variable) with a short-hand within the template:

{{#let name=person.bio.firstName color=generateColor}}
  <div>{{name}} gets a {{color}} card!</div>
{{/let}}

Variables introduced this way take precedence over names of templates, global helpers, fields of the current data context and previously introduced variables with the same name.

Custom Block Helpers

To define your own block helper, simply declare a template, and then invoke it using {{#someTemplate}} (block) instead of {{> someTemplate}} (inclusion) syntax.

When a template is invoked as a block helper, it can use {{> Template.contentBlock}} and {{> Template.elseBlock}} to include the block content it was passed.

Here is a simple block helper that wraps its content in a div:

<template name="note">
  <div class="note">
    {{> Template.contentBlock}}
  </div>
</template>

You would invoke it as:

{{#note}}
  Any content here
{{/note}}

Here is an example of implementing #unless in terms of #if (ignoring for the moment that unless is a built-in directive):

<template name="unless">
  {{#if this}}
    {{> Template.elseBlock}}
  {{else}}
    {{> Template.contentBlock}}
  {{/if}}
</template>

Note that the argument to #unless (the condition) becomes the data context in the unless template and is accessed via this. However, it would not work very well if this data context was visible to Template.contentBlock, which is supplied by the user of unless.

Therefore, when you include {{> Template.contentBlock}}, Spacebars hides the data context of the calling template, and any data contexts established in the template by #each and #with. They are not visible to the content block, even via ... Put another way, it's as if the {{> Template.contentBlock}} inclusion occurred at the location where {{#unless}} was invoked, as far as the data context stack is concerned.

You can pass an argument to {{> Template.contentBlock}} or {{> Template.elseBlock}} to invoke it with a data context of your choice. You can also use {{#if Template.contentBlock}} to see if the current template was invoked as a block helper rather than an inclusion.

Comment Tags

Comment template tags begin with {{! and can contain any characters except for }}. Comments are removed upon compilation and never appear in the compiled template code or the generated HTML.

{{! Start of a section}}
<div class="section">
  ...
</div>

Comment tags also come in a "block comment" form. Block comments may contain {{ and }}:

{{!-- This is a block comment.
We can write {{foo}} and it doesn't matter.
{{#with x}}This code is commented out.{{/with}}
--}}

Comment tags can be used wherever other template tags are allowed.

Nested sub-expressions

Sometimes an argument to a helper call is best expressed as a return value of some other expression. For this and other cases, one can use parentheses to express the evaluation order of nested expressions.

{{capitalize (getSummary post)}}

In this example, the result of the getSummary helper call will be passed to the capitalize helper.

Sub-expressions can be used to calculate key-word arguments, too:

{{> tmpl arg=(helper post)}}

HTML Dialect

Spacebars templates are written in standard HTML extended with additional syntax (i.e. template tags).

Spacebars validates your HTML as it goes and will throw a compile-time error if you violate basic HTML syntax in a way that prevents it from determining the structure of your code.

Spacebars is not lenient about malformed markup the way a web browser is. While the latest HTML spec standardizes how browsers should recover from parse errors, these cases are still not valid HTML. For example, a browser may recover from a bare < that does not begin a well-formed HTML tag, while Spacebars will not. However, gone are the restrictions of the XHTML days; attribute values do not have to quoted, and tags are not case-sensitive, for example.

You must close all HTML tags except the ones specified to have no end tag, like BR, HR, IMG and INPUT. You can write these tags as <br> or equivalently <br/>.

The HTML spec allows omitting some additional end tags, such as P and LI, but Spacebars doesn't currently support this.

Top-level Elements in a .html file

Technically speaking, the <template> element is not part of the Spacebars language. A foo.html template file in Meteor consists of one or more of the following elements:

• <template name="myName"> - The <template> element contains a Spacebars template (as defined in the rest of this file) which will be compiled to the Template.myName component.

• <head> - Static HTML that will be inserted into the <head> element of the default HTML boilerplate page. Cannot contain template tags. If <head> is used multiple times (perhaps in different files), the contents of all of the <head> elements are concatenated.

• <body> - A template that will be inserted into the <body> of the main page. It will be compiled to the Template.body component. If <body> is used multiple times (perhaps in different files), the contents of all of the <body> elements are concatenated.

Escaping Curly Braces

To insert a literal {{, {{{, or any number of curly braces, put a vertical bar after it. So {{| will show up as {{, {{{| will show up as {{{, and so on.

underscore

Underscore is a utility-belt library for JavaScript that provides support for functional programming. It is invaluable for writing clear, concise JavaScript in a functional style.

The underscore package defines the _ namespace on both the client and the server.

webapp

The webapp package is what lets your Meteor app serve content to a web browser. It is included in the meteor-base set of packages that is automatically added when you run meteor create. You can easily build a Meteor app without it - for example if you wanted to make a command-line tool that still used the Meteor package system and DDP.

This package also allows you to add handlers for HTTP requests. This lets other services access your app's data through an HTTP API, allowing it to easily interoperate with tools and frameworks that don't yet support DDP.

webapp exposes the connect API for handling requests through WebApp.connectHandlers. Here's an example that will let you handle a specific URL:

// Listen to incoming HTTP requests, can only be used on the server
WebApp.connectHandlers.use("/hello", function(req, res, next) {
  res.writeHead(200);
  res.end("Hello world from: " + Meteor.release);
});

WebApp.connectHandlers.use([path], handler) has two arguments:

path - an optional path field. This handler will only be called on paths that match this string. The match has to border on a / or a .. For example, /hello will match /hello/world and /hello.world, but not /hello_world.

handler - this is a function that takes three arguments:

• req - a Node.js IncomingMessage object with some extra properties. This argument can be used to get information about the incoming request.
• res - a Node.js ServerResponse object. Use this to write data that should be sent in response to the request, and call res.end() when you are done.
• next - a function. Calling this function will pass on the handling of this request to the next relevant handler.