TypeStript 入门 - 05：类型保护

typeof 类型保护

function double(val: number | string) {
    if (typeof val === 'number') {
        val
    } else {
        val
    }
}

instanceof类型保护

class Cat { }
class Dog { }

//说明：clazz 拥有 new() 方法
const getInstance = (clazz: { new(): Cat | Dog }) => {
    return new clazz();
}

let r = getInstance(Cat);
if(r instanceof Cat){
    r
}else{
    r
}

in类型保护

interface Fish {
 swiming: string,
}

interface Bird {
 fly: string,
 leg: number
}

function isBird(animal: Fish | Bird): animal is Bird {
 return 'fly' in animal && 'leg' in animal
}

function getAniaml(animal: Fish | Bird) {
 if (isBird(animal)) {
  console.log("isBird");
 } else {
  console.log("Fish");
 }
}

getAniaml({ "swiming": "x" });
//output: Fish

可辨识联合类型

interface WarningButton {
 class: 'warning'
}
interface DangerButton {
 class: 'danger'
}
function createButton(button: WarningButton | DangerButton) {
 if (button.class == 'warning') {
  console.log("Warning Button");
 } else {
  console.log("Warning Button");
 }
}
createButton({ class: 'warning' });
//output: Warning Button

null保护

const addPrefix = (num?: number) => {
 num = num || 1.1;
 function prefix(fix: string) {
  //利用?.进行保护
  return fix + num?.toFixed()
 }
 return prefix('xx');
}
console.log(addPrefix());
//output: xx.1

完整性保护

interface ICircle {
 kind: 'circle',
 r: number
}
interface IRant {
 kind: 'rant',
 width: number,
 height: number
}
interface ISquare {
 kind: 'square',
 width: number
}
type Area = ICircle | IRant | ISquare
const isAssertion = (obj: never) => { }
const getArea = (obj: Area) => {
 switch (obj.kind) {
  case 'circle':
   return 3.14 * obj.r ** 2
  default:
   // 不可抵达的断言保护, 此分支逻辑必须实现, 否则报错
   return isAssertion(obj);
 }
}