錯誤之處，歡迎指正。

1. 類型約束

• number、string、boolean

let num: number = 1;
let str: string = "123";
let bol: boolean = true;
num = str; // Error

let id: any = "123";
id = 1;
id = true;
console.log(id); // true

• Array

const arr: Array<number> = [
  1, 2, 3
];
const arr2: Array<string> = [
  1
]; // Error

• type

type a = 1;
const num: a = 1;
const num1: a = 2; // Error
const num2: a = "1"; // Error

• interface

interface itemModel {
  name: string,
  age: number
};
const obj: itemModel = {
  name: "chris",
  age: 17
};
// 使用在函數(shù)參數(shù)中
interface obj {
  name: string,
  age: number
} ;
function foo({name, age}: obj) {};
foo({
  name: "chris",
  age: 17
});
// 對函數(shù)進行約束
interface foo {
  (num: number): number
};
let myFoo: foo = (a) => { return a + "" }; // Error
myFoo(1);

• void
  表示函數(shù)的返回值為undefined。

function foo(): void {
  return true; // Error
};

2. 斷言

• as

const app: HTMLElement = document.getElementById("root"); // Error

const app: HTMLElement = document.getElementById("root") as HTMLElement;
const app: HTMLElement | null = document.getElementById("root");