Modular Libraries Some libraries only work in a module loader environment. For example, because express only works in Node.js and must be loaded using the CommonJS require function. ECMAScript 2015 (also known as ES2015, ECMAScript 6, and ES6), CommonJS, and RequireJS have similar notions of importing a module. In JavaScript CommonJS (Node.js), for example, you would write var fs = require("fs"); In TypeScript or ES6, the import keyword serves the same purpose: import fs = require("fs"); You’

Readonly properties Some properties should only be modifiable when an object is first created. You can specify this by putting readonly before the name of the property: interface Point { readonly x: number; readonly y: number; } You can construct a Point by assigning an object literal. After the assignment, x and y can’t be changed. let p1: Point = { x: 10, y: 20 }; p1.x = 5; // error! TypeScript comes with a ReadonlyArray<T> type that is the same as Array<T> with all mutating

Best common type When a type inference is made from several expressions, the types of those expressions are used to calculate a “best common type”. For example, let x = [0, 1, null]; To infer the type of x in the example above, we must consider the type of each array element. Here we are given two choices for the type of the array: number and null. The best common type algorithm considers each candidate type, and picks the type that is compatible with all the other candidates. Because the best

var declarations Declaring a variable in JavaScript has always traditionally been done with the var keyword. var a = 10; As you might’ve figured out, we just declared a variable named a with the value 10. We can also declare a variable inside of a function: function f() { var message = "Hello, world!"; return message; } and we can also access those same variables within other functions: function f() { var a = 10; return function g() { var b = a + 1; return b; } } var g =

Void void is a little like the opposite of any: the absence of having any type at all. You may commonly see this as the return type of functions that do not return a value: function warnUser(): void { alert("This is my warning message"); } Declaring variables of type void is not useful because you can only assign undefined or null to them: let unusable: void = undefined;

// Type definitions for [~THE LIBRARY NAME~] [~OPTIONAL VERSION NUMBER~] // Project: [~THE PROJECT NAME~] // Definitions by: [~YOUR NAME~] <[~A URL FOR YOU~]> /*~ This is the module template file. You should rename it to index.d.ts *~ and place it in a folder with the same name as the module. *~ For example, if you were writing a file for "super-greeter", this *~ file should be 'super-greeter/index.d.ts' */ /*~ If this module is a UMD module that exposes a global variable 'myLib' wh

Create a webpack configuration file Create a webpack.config.js file at the root of the project directory. module.exports = { entry: "./src/index.tsx", output: { filename: "./dist/bundle.js", }, // Enable sourcemaps for debugging webpack's output. devtool: "source-map", resolve: { // Add '.ts' and '.tsx' as resolvable extensions. extensions: ["", ".webpack.js", ".web.js", ".ts", ".tsx", ".js"] }, module: { loaders: [ // All files with a '.ts' or '.tsx' ex

Variable capturing quirks Take a quick second to guess what the output of the following snippet is: for (var i = 0; i < 10; i++) { setTimeout(function() { console.log(i); }, 100 * i); } For those unfamiliar, setTimeout will try to execute a function after a certain number of milliseconds (though waiting for anything else to stop running). Ready? Take a look: 10 10 10 10 10 10 10 10 10 10 Many JavaScript developers are intimately familiar with this behavior, but if you’re surprised, you’r

Create a new project Choose File Choose New Project Choose Visual C# Choose ASP.NET Web Application Choose MVC I unchecked “Host in the cloud” since this will be a local demo. Run the application and make sure that it works.

Abstract Classes Abstract classes are base classes from which other classes may be derived. They may not be instantiated directly. Unlike an interface, an abstract class may contain implementation details for its members. The abstract keyword is used to define abstract classes as well as abstract methods within an abstract class. abstract class Animal { abstract makeSound(): void; move(): void { console.log("roaming the earth..."); } } Methods within an abstract class that are marked