TypeScript

Cheatsheet

Types

Assigning Variables

//Number
let x = 42;
let exampleNumber:number = 66;

//String 
let exampleName: string = "Person1";

//Boolean
let exampleBoolean: boolean = false;

//Date
let exampleDate: Date = new Date(2021, 7, 17);

//Any
let exampleAny: any = "Can be anything";

//Array
let exampleArray1: string[] = ['index1', 'index2'];
let exampleArray2: Array<number> = [1,3];
let exampleArray3: number[] = [2,4];

//Enum
enum color{
 RED,
 GREEN,
 BLUE,
}
let exampleEnum: color = color.GREEN;
console.log('exampleEnum :: ${typeof exampleEnum}');

//Null
let exampleNull: number = null;

//Tuple
//Similar to an array but different indexes have different types
let exampleTuple: [string, number];

Assigning Constant

//const can be used instead of let in above types
const exampleConstNum: number = 99;
const exampleConstString: string = "Value";

Void Function

function exampleVoid(msg: string): void {
console.exampleVoid(msg);
}

Classes

Example class:

class OrderLogic {
    constructor(public order: IOrder) { }
    getOrderTotal(): number {
        let sum: number = 0;
        for (let orderDetail of
this.order.orderDetails)
        {
            sum += orderDetail.price;
        }
        return sum;
    }
}

Installation

npm install TypeScript

Compiler comments

Don’t check this file

// @ts-nocheck

Check this file (JS)

// @ts-check

Ignore the next line

// @ts-ignore

Expect an error on the next line

// @ts-expect-error

Operators

&&=

let a;

let b = 1;
 
// assign a value only if current value is truthy
 
a &&= 'default'; // a is still undefined

b &&=  5; // b is now 5

||=

let a;

let b = 1;
 
// assign a value only if current value is false
 
a ||= 'default'; // a is 'default' now

b ||=  5; // b is still 1

??=

let a;

let b = 0;
 
// assign a value only if current value is null or undefined
 
a ??= 'default'; // a is now 'default'

b ??=  5; // b is still 0