Strong typing for JavaScript.

By default in JavaScript, you can access any property or call any function on any variable. The only problem is that you might get a runtime error. TypeScript adds strong typing to detect and avoid these issues at the build level.

Compile TypeScript

tsc hello.ts will "compile" the .ts file and create a .js file.

Two stricter compiler flags:

• noImplicitAny - won't fall back to the most lenient type (any) as this partially defeats the point of TS.
• strictNullChecks - null and undefined are used to signal absent or uninitialised values, and can be assigned to any types

I used Grunt to help automate the build process - see [grunt-typescript](<https://www.npmjs.com/package/grunt-typescript>) and set up your tsconfig.json

• Example tsconfig.json

Everyday types

The primitive types are string, number, and boolean.

You can signify arrays using number[] or Array<number> (the latter is for generics).

any is a special type that can be used to avoid typechecking errors - you can access any property, call it like a function, or assign it to/from anything.

Type Annotations are done like Go: let myName: string = "Will"

But you don't need them in most cases - it will infer the types.

Functions can have type annotations after each parameter and for a return value

function greet(name: string) {

function multiply(value: number): number {

Typically a return annotation isn't needed, as it can be inferred.

Objects can also be typed. When passed as a function parameter, you can just list its properties and their types:

function printCoord(pt: { x: number; y: number }) {

Optional properties are also valid and Swift-like. It means values can be undefined.

Union types allow you to represent any one of a given type:

function printId(id: number | string) {

But then to actually use id, you have to narrow the type using if statements such as if (typeof id === "string") or if (Array.isArray(x))

More useful stuff

Type Aliases let you define a type one and refer to it by name. It works a bit like a macro.

type Point = {
	x: number;
	y: number;
};

function printCoord(pt: Point) {
	...