Type Aliases Type aliases create a new name for a type. Type aliases are sometimes similar to interfaces, but can name primitives, unions, tuples, and any other types that you’d otherwise have to write by hand. type Name = string; type NameResolver = () => string; type NameOrResolver = Name | NameResolver; function getName(n: NameOrResolver): Name { if (typeof n === "string") { return n; } else { return n(); } } Aliasing doesn’t actually create a new type - it creates a new

Any We may need to describe the type of variables that we do not know when we are writing an application. These values may come from dynamic content, e.g. from the user or a 3rd party library. In these cases, we want to opt-out of type-checking and let the values pass through compile-time checks. To do so, we label these with the any type: let notSure: any = 4; notSure = "maybe a string instead"; notSure = false; // okay, definitely a boolean The any type is a powerful way to work with existin

The as operator Recall how to write a type assertion: var foo = <foo>bar; Here we are asserting the variable bar to have the type foo. Since TypeScript also uses angle brackets for type assertions, JSX’s syntax introduces certain parsing difficulties. As a result, TypeScript disallows angle bracket type assertions in .tsx files. To make up for this loss of functionality in .tsx files, a new type assertion operator has been added: as. The above example can easily be rewritten with the as

Write some code Let’s write our first TypeScript file using Knockout. First, create a new file in your src directory named hello.ts. import * as ko from "knockout"; class HelloViewModel { language: KnockoutObservable<string> framework: KnockoutObservable<string> constructor(language: string, framework: string) { this.language = ko.observable(language); this.framework = ko.observable(framework); } } ko.applyBindings(new HelloViewModel("TypeScript", "Knockout")); N

Interfaces Extending Classes When an interface type extends a class type it inherits the members of the class but not their implementations. It is as if the interface had declared all of the members of the class without providing an implementation. Interfaces inherit even the private and protected members of a base class. This means that when you create an interface that extends a class with private or protected members, that interface type can only be implemented by that class or a subclass of

Global Plugin A global plugin is global code that changes the shape of some global. As with global-modifying modules, these raise the possibility of runtime conflict. For example, some libraries add new functions to Array.prototype or String.prototype.

Symbol.search A regular expression method that returns the index within a string that matches the regular expression. Called by the String.prototype.search method.

Install TypeScript If your version of Visual Studio does not already have TypeScript, you can install it for Visual Studio 2015 or Visual Studio 2013. This quickstart uses Visual Studio 2015.

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Export statements Export statements are handy when exports need to be renamed for consumers, so the above example can be written as: class ZipCodeValidator implements StringValidator { isAcceptable(s: string) { return s.length === 5 && numberRegexp.test(s); } } export { ZipCodeValidator }; export { ZipCodeValidator as mainValidator };