Object destructuring You can also destructure objects: let o = { a: "foo", b: 12, c: "bar" } let {a, b} = o; This creates new variables a and b from o.a and o.b. Notice that you can skip c if you don’t need it. Like array destructuring, you can have assignment without declaration: ({a, b} = {a: "baz", b: 101}); Notice that we had to surround this statement with parentheses. JavaScript normally parses a { as the start of block.

Number As in JavaScript, all numbers in TypeScript are floating point values. These floating point numbers get the type number. In addition to hexadecimal and decimal literals, TypeScript also supports binary and octal literals introduced in ECMAScript 2015. let decimal: number = 6; let hex: number = 0xf00d; let binary: number = 0b1010; let octal: number = 0o744;

Null and Undefined In TypeScript, both undefined and null actually have their own types named undefined and null respectively. Much like void, they’re not extremely useful on their own: // Not much else we can assign to these variables! let u: undefined = undefined; let n: null = null; By default null and undefined are subtypes of all other types. That means you can assign null and undefined to something like number. However, when using the --strictNullChecks flag, null and undefined are only

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Never The never type represents the type of values that never occur. For instance, never is the return type for a function expression or a arrow function expresssion that always throws an exception or one that never returns; Variables also acquire the type never when narrowed by any type guards that can never be true. The never type is a subtype of, and assignable to, every type; however, no type is a subtype of, or assignable to, never (except never itself). Even any isn’t assignable to never.

Needless Namespacing If you’re converting a program from namespaces to modules, it can be easy to end up with a file that looks like this: shapes.ts export namespace Shapes { export class Triangle { /* ... */ } export class Square { /* ... */ } } The top-level module here Shapes wraps up Triangle and Square for no reason. This is confusing and annoying for consumers of your module: shapeConsumer.ts import * as shapes from "./shapes"; let t = new shapes.Shapes.Triangle(); // shapes.Sha

Namespacing As we add more validators, we’re going to want to have some kind of organization scheme so that we can keep track of our types and not worry about name collisions with other objects. Instead of putting lots of different names into the global namespace, let’s wrap up our objects into a namespace. In this example, we’ll move all validator-related entities into a namespace called Validation. Because we want the interfaces and classes here to be visible outside the namespace, we preface

Namespaced Validators namespace Validation { export interface StringValidator { isAcceptable(s: string): boolean; } const lettersRegexp = /^[A-Za-z]+$/; const numberRegexp = /^[0-9]+$/; export class LettersOnlyValidator implements StringValidator { isAcceptable(s: string) { return lettersRegexp.test(s); } } export class ZipCodeValidator implements StringValidator { isAcceptable(s: string) { return s.length === 5 && numberRegexp.test(s); }

Multi-file namespaces Here, we’ll split our Validation namespace across many files. Even though the files are separate, they can each contribute to the same namespace and can be consumed as if they were all defined in one place. Because there are dependencies between files, we’ll add reference tags to tell the compiler about the relationships between the files. Our test code is otherwise unchanged. Validation.ts namespace Validation { export interface StringValidator { isAcceptable(s: str

MSBuild Update project file to include locally installed Microsoft.TypeScript.Default.props (at the top) and Microsoft.TypeScript.targets (at the bottom) files: <?xml version="1.0" encoding="utf-8"?> <Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003"> <!-- Include default props at the bottom --> <Import Project="$(MSBuildExtensionsPath32)\Microsoft\VisualStudio\v$(VisualStudioVersion)\TypeScript\Microsoft.T