Writing Helpers

Writing Helpers

Helpers allow you to add additional functionality to your templates beyond what is included out-of-the-box in Ember. Helpers are most useful for transforming raw values from models and components into a format more appropriate for your users.

For example, imagine we have an Invoice model that contains a totalDue attribute, which represents the total amount due for that invoice. Because we do not want our company to go out of business due to strange JavaScript rounding errors, we store this value in cents instead of a floating point dollar value.

However, if we display dollar values to our users as "100¢" instead of "$1.00", they may be very confused. We can write a helper to format these values into the appropriate human-readable form.

Let's create a format-currency helper that takes an integer count of cents and turns it into formatted dollars.

To use the format-currency helper, you call it using curly braces in your template:

Your total is {{format-currency model.totalDue}}.

Let's now implement the helper. Helpers are functions that take one or more inputs and return a single output that should be put into the HTML.

To add a new helper, create a file with the name of the helper you want (e.g. format-currency.js) in your application's helpers directory. You can also have Ember generate the file for you from the command line:

ember generate helper format-currency

That file should export a function wrapped with Ember.Helper.helper():

import Ember from 'ember';

export function formatCurrency([value, ...rest]) {
  let dollars = Math.floor(value / 100);
  let cents = value % 100;
  let sign = '$';

  if (cents.toString().length === 1) { cents = '0' + cents; }
  return `${sign}${dollars}.${cents}`;
}

export default Ember.Helper.helper(formatCurrency);

In this example, the function receives a dollar amount in cents as the first parameter (value). We then use regular JavaScript to turn the count of cents into a formatted string, like "$5.00".

Whenever you use your helper in a template, Ember will call this function and insert whatever you return from the helper into the DOM.

So, if we want to display a purchase total we can pass the value into the template in cents:

Your total is {{format-currency 250}}.

And Ember makes use of our new helper function to replace the content inside the {{ }} with the formatted amount.

Your total is $2.50.

Whenever the arguments you've passed to a helper change, whether they come from a model or a component, Ember will automatically call your helper again with the new values and keep the page up-to-date.

Helper Names

Unlike components, which require a dash in the name to follow the Custom Element spec, helper names can be single or multi-word. If your helper's name is multi-word, it should be dasherized as the examples on this page.

Helper Arguments

You can pass one or more arguments to be used inside the function. In the above example, we passed the amount in cents as the first and only argument.

To pass multiple arguments to a helper, add them as a space-separated list after the helper name:

{{my-helper "hello" "world"}}

An array of these arguments is passed to the helper function:

import Ember from 'ember';

export default Ember.Helper.helper(function(params) {
  let [arg1, arg2] = params;

  console.log(arg1); // => "hello"
  console.log(arg2); // => "world"
});

You can use JavaScript's destructuring assignment shorthand to clean up the code. This example is equivalent to the above example (note the function signature):

import Ember from 'ember';

export default Ember.Helper.helper(function([arg1, arg2]) {
  console.log(arg1); // => "hello"
  console.log(arg2); // => "world"
});

Named Arguments

Normal arguments are useful for passing data to be transformed into helper functions. However, because the order in which you pass arguments matters, it is usually best not to have helpers take more than one or two of them.

That said, sometimes you may want to make behavior of helpers configurable by the developers that call them from their templates. For example, let's abandon our Americentric ways and update our format-currency helper to take an optional configuration for which currency symbol to display.

Helpers allow you to pass named arguments as a JavaScript object that contains the name of the argument along with an associated value. The order in which named arguments are supplied does not affect functionality.

In this example, we can pass a sign argument to our format-currency helper:

{{format-currency 350 sign="£"}}

We'd like our helper to print pounds sterling rather than US dollars:

£3.50

The object containing named arguments is passed as the second argument to the helper function. Here is our example from above, updated to support the optional sign option:

import Ember from 'ember';

export default Ember.Helper.helper(function([value, ...rest], namedArgs) {
  let dollars = Math.floor(value / 100);
  let cents = value % 100;
  let sign = namedArgs.sign === undefined ? '$' : namedArgs.sign;

  if (cents.toString().length === 1) { cents = '0' + cents; }
  return `${sign}${dollars}.${cents}`;
});

You can pass as many named arguments as you'd like. They get added to the namedArgs argument passed to the function:

{{my-helper option1="hello" option2="world" option3="goodbye cruel world"}}

import Ember from 'ember';

export default Ember.Helper.helper(function(params, namedArgs) {
  console.log(namedArgs.option1); // => "hello"
  console.log(namedArgs.option2); // => "world"
  console.log(namedArgs.option3); // => "goodbye cruel world"
});

You can use JavaScript's destructuring assignment shorthand in this case as well to clean up the above code:

import Ember from 'ember';

export default Ember.Helper.helper(function(params, { option1, option2, option3 }) {
  console.log(option1); // => "hello"
  console.log(option2); // => "world"
  console.log(option3); // => "goodbye cruel world"
});

In sum, arguments are good for passing values:

{{format-date currentDate}}

Hashes are useful for configuring the behavior of a helper:

{{print-current-date format="YYYY MM DD"}}

You can have as many of both as you want, so long as the parameters come first:

{{format-date-and-time date time format="YYYY MM DD h:mm" locale="en"}}

The above example contains two arguments:

• date
• time

And two named arguments:

• format="YYY MM DD h:mm"
• locale="en"

Class-based Helpers

By default, helpers are stateless. They are passed inputs (parameters and a hash), they perform an operation on those inputs, and return a single output. They have no side-effects and don't save any information that is used on subsequent runs of the function.

In some situations, however, you may need to write a helper that interacts with the rest of your application. You can create class-based helpers that have access to services in your application, and can optionally save state as well, although this is usually unnecessary and error-prone.

To create a class-based helper, rather than exporting a simple function, you should export a subclass of Ember.Helper. Helper classes must contain a compute method that behaves the same as the function passed to Ember.Helper.helper. In order to access a service, you must first inject it into the class-based helper. Once added, you can call the service's methods or access its properties from within the compute() method.

As an exercise, here is the above format-currency helper re-factored into a class-based helper:

import Ember from 'ember';

export default Ember.Helper.extend({
  compute([value, ...rest], hash) {
    let dollars = Math.floor(value / 100);
    let cents = value % 100;
    let sign = hash.sign === undefined ? '$' : hash.sign;

    if (cents.toString().length === 1) { cents = '0' + cents; }
    return `${sign}${dollars}.${cents}`;
  }
});

This is exactly equivalent to the format-currency example above. You can think of the function version as a shorthand for the longer class form if it does not require dependency injection.

As another example, let's make a helper utilizing an authentication service that welcomes users by their name if they're logged in:

import Ember from 'ember';

export default Ember.Helper.extend({
  authentication: Ember.inject.service(),
  compute() {
    let authentication = this.get('authentication');

    if (authentication.get('isAuthenticated')) {
      return 'Welcome back, ' + authentication.get('username');
    } else {
      return 'Not logged in';
    }
  }
});

Escaping HTML Content

To protect your application from cross-site scripting attacks (XSS), Ember automatically escapes any value you return from a helper so that the browser will not interpret it as HTML.

For example, here's a make-bold helper that returns a string containing HTML:

import Ember from 'ember';

export default Ember.Helper.helper(function([param, ...rest]) {
  return `<b>${param}</b>`;
});

You can invoke it like this:

{{make-bold "Hello world"}}

Ember will escape the HTML tags, like this:

<b>Hello world</b>

This shows the literal string <b>Hello world</b> to the user, rather than the text in bold as you probably intended. We can tell Ember not to escape the return value (that is, that it is safe) by using the htmlSafe string utility:

import Ember from 'ember';

export default Ember.Helper.helper(function([param, ...rest]) {
  return Ember.String.htmlSafe(`<b>${param}</b>`);
});

If you return a SafeString (a string that has been wrapped in a call to htmlSafe), Ember knows that you have vouched on its behalf that it contains no malicious HTML.

However, note that in the above code we may have inadvertently introduced an XSS vulnerability into our application! By blindly marking the string as safe, a malicious user could get their own HTML into our app, allowing them to do things like access sensitive customer data.

For example, imagine that we have a chat app and use our make-bold helper to welcome the new users into the channel:

Welcome back! {{make-bold model.firstName}} has joined the channel.

Now a malicious user simply needs to set their firstName to a string containing HTML (like a <script> tag that sends private customer data to their server, for example) and every user in that chat room has been compromised.

In general, you should prefer using components if you are wrapping content in HTML. However, if you really want to include a mix of HTML and values from models in what you return from the helper, make sure you escape anything that may have come from an untrusted user with the escapeExpression utility:

import Ember from 'ember';

export default Ember.Helper.helper(function([param, ...rest]) {
  let value = Ember.Handlebars.Utils.escapeExpression(param);
  return Ember.String.htmlSafe(`<b>${value}</b>`);
});

Now the value passed into the helper has its HTML escaped, but the trusted <b> tags that we want to wrap the value in are not escaped. A malicious user setting their firstName to something containing HTML would see this:

Welcome back! <b>&lt;script
type="javascript"&gt;alert('pwned!');&lt;/script&gt;</b> has joined the channel.