/wiki/typescript

Superset of JavaScript, can be learned incrementally.

Can be transpiled to multiple JavaScript flavors.

Type annotations & Strongly typed libraries, code is automatically documented in the IDE. Refer to online documentation for libraries that you use. The compiler can catch bugs in advance...

Resources

Compiler

tsc - the compiler tsc index.ts transpiles index.ts into index.js

config

tsconfig.json is the standard way to configure the compiler, and is automatically picked up upon running the tsc command.

Layout

{
    "compilerOptions": {
        "target": "esnext",
        "watch": true,
        "lib": ["dom", "es2017"]
    }
}

lib - Automatically includes typings for certain environments such as the dom, or es2017. What you set in this field gives you access to the classes in whatever library you've designated and provides you with integrated documentation, intellisense, autocomplete, and if there are errors, it will inform you as to why this code will not run.

If you need to do deep inspection of code, you can right click and view the typings which will show you a view of every property & method that exists on that class.

Monorepo

Community maintained types availble for libraries without them, npm i -D @types/packagename

Strong typing code

Implicitly and explicitly.

Example of implicit.

let hello = 5;

Given a number, this type is already inferred as a primitive number type.

We can opt out of the type system by annotating variables with, any. The compiler will not type check this.

let hello: any = 5;

Without any type annoations, it's inferred as an any type. So you can assign anything to it.

let hello;

Functions

function pow(x: number, y: number): string {
    return Math.pow(x, y).toString()
}

So where x and y are both ints, the return type is string. Therefore, we must convert the return toString() in order for this code to run.

Void is also acceptable for things like event listeners or functions that just don't return a value

Arrays

const arr: number[] = []

arr.push(1)
arr.push('23') -- error
arr.push(false) -- error

Tuples

Fixed length arrays that each value has its own type.

type MyList = [number?, string?, boolean?]

Generics

class Observable<T> {

}

The capital T represents a variable type that we can pass in to strong type observables internal value.

let x: Observable<number>

let y: Observable<Person>