珠峰架构师课

9.类型保护

通过判断识别所执行的代码块，自动识别变量属性和方法

一.typeof类型保护

function double(val: number | string) {
    if (typeof val === 'number') {
        val
    } else {
        val
    }
}
1
2
3
4
5
6
7

二.instanceof类型保护

class Cat { }
class Dog { }

const getInstance = (clazz: { new(): Cat | Dog }) => {
    return new clazz();
}
let r = getInstance(Cat);
if(r instanceof Cat){
    r
}else{
    r
}
1
2
3
4
5
6
7
8
9
10
11
12

三.in类型保护

interface Fish {
    swiming: string,
}
interface Bird {
    fly: string,
    leg: number
}
function getType(animal: Fish | Bird) {
    if ('swiming' in animal) {
        animal // Fish
    } else {
        animal // Bird
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14

四.可辨识联合类型

interface WarningButton {
    class: 'warning'
}
interface DangerButton {
    class: 'danger'
}
function createButton(button: WarningButton | DangerButton) {
    if (button.class == 'warning') {
        button // WarningButton
    } else {
        button // DangerButton
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13

五.null保护

const addPrefix = (num?: number) => {
    num = num || 1.1;
    function prefix(fix: string) {
        return fix + num?.toFixed()
    }
    return prefix('zf');
}
console.log(addPrefix());
1
2
3
4
5
6
7
8

这里要注意的是ts无法检测内部函数变量类型

六.自定义类型保护

interface Fish {
    swiming: string,
}
interface Bird {
    fly: string,
    leg: number
}
function isBird(animal: Fish | Bird):animal is Bird {
    return 'swiming' in animal
}
function getAniaml (animal:Fish | Bird){
    if(isBird(animal)){
        animal
    }else{
        animal
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

七.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:
            return isAssertion(obj); // 必须实现所有逻辑
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

← 10.类型推断 →