类型守卫

in 关键字

interface Admin {
    name: string
    privileges: string[]
}

interface Employee {
    name: string
    startDate: Date
}

type UnknownEmployee = Employee | Admin

function printEmployeeInformation(emp: UnknownEmployee) {
    console.log('Name: ' + emp.name)
    if ('privileges' in emp) {
        console.log('Privileges: ' + emp.privileges)
    }
    if ('startDate' in emp) {
        console.log('Start Date: ' + emp.startDate)
    }
}

typeof 关键字

function padLeft(value: string, padding: string | number) {
    if (typeof padding === 'number') {
        return Array(padding + 1).join(' ') + value
    }
    if (typeof padding === 'string') {
        return padding + value
    }
    throw new Error(`Expected string or number, got '${padding}'.`)
}

typeof 类型保护只支持两种形式：typeof v === "typename" 和 typeof v !== typename，"typename" 必须是 "number"， "string"， "boolean" 或 "symbol"。 但是 TypeScript 并不会阻止你与其它字符串比较，语言不会把那些表达式识别为类型保护。

instanceof 关键字

interface Padder {
    getPaddingString(): string
}

class SpaceRepeatingPadder implements Padder {
    constructor(private numSpaces: number) {}
    getPaddingString() {
        return Array(this.numSpaces + 1).join(' ')
    }
}

class StringPadder implements Padder {
    constructor(private value: string) {}
    getPaddingString() {
        return this.value
    }
}

let padder: Padder = new SpaceRepeatingPadder(6)

if (padder instanceof SpaceRepeatingPadder) {
    // padder的类型收窄为 'SpaceRepeatingPadder'
}

自定义类型保护

function isNumber(x: any): x is number {
  return typeof x === "number";
}

function isString(x: any): x is string {
  return typeof x === "string";
}