TypeScript Tutorial

1. Installing TypeScript and Demo

Enter the command line and following command:

npm install -g typescript
tsc -V

Compile the ts file:

tsc xxx.ts

ts demo

console.log('Hello TS')

tsc hello.ts 

2. Type Declaration

// Declare variables and specify types
let a: number;
a = 10;
a = 33;
// a = 'hello'; // error TS2322: Type 'string' is not assignable to type 'number'.

let b: string;
b = 'hello';
// b = 123; // error TS2322: Type 'number' is not assignable to type 'string'.

// Declare variables and assign values
let c: boolean = true;

// If a variable is declared and assigned, TS can automatically detect the type of the variable
let d = true;
// c = 123; // error TS2322: Type 'number' is not assignable to type 'boolean'.

// JavaScript does not consider the type and number of parameters
// function sum(a , b){
//     return a + b;
// }
//
// sum(123, 456); //  579
// sum(123, "456"); //  123456

// a is number, b is number, return val is number
function sum(a: number, b: number): number {
    return a + b;
}

sum(123, 456);
// sum(123, "456"); // error TS2345: Argument of type 'string' is not assignable to parameter of type 'number'.
// sum(123, 456, 789); // error TS2554: Expected 2 arguments, but got 3.