给出一大些面试题,然后不给答案。前端本来就卷,一些面试官看了文章后可能在自己都不知道答案的情况下,就跑去问面试者。我其实挺反感的这类文章的。
ReadonlyArray<number>
声明的数组不能使用push,pop等方法。枚举会被编译时会编译成一个对象,可以被当作对象使用
const枚举会在ts编译期间被删除,避免额外的性能开销
// 普通枚举
enum Witcher {
Ciri = 'Queen',
Geralt = 'Geralt of Rivia'
}
function getGeraltMessage(arg: {[key: string]: string}): string {
return arg.Geralt
}
getGeraltMessage(Witcher) // Geralt of Rivia
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// const枚举
const enum Witcher {
Ciri = 'Queen',
Geralt = 'Geralt of Rivia'
}
const witchers: Witcher[] = [Witcher.Ciri, Witcher.Geralt]
// 编译后
// const witchers = ['Queen', 'Geralt of Rivia'
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// 函数类型
interface SearchFunc {
(source: string, subString: string): boolean;
}
let mySearch: SearchFunc;
mySearch = function(source: string, subString: string) {
let result = source.search(subString);
return result > -1;
}
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// Array
interface StringArray {
[index: number]: string;
}
let myArray: StringArray;
myArray = ["Bob", "Fred"];
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// Class, constructor存在于类的静态部分,所以不会检查
interface ClockInterface {
currentTime: Date;
setTime(d: Date);
}
class Clock implements ClockInterface {
currentTime: Date;
setTime(d: Date) {
this.currentTime = d;
}
constructor(h: number, m: number) { }
}
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不了解
type Name = { name: string }
type Age = { age: number }
type Union = Name | Age
type UnionKey<P> = P extends infer P ? keyof P : never
type T = UnionKey<Union>
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// ??
let x = foo ?? bar();
// 等价于
let x = foo !== null && foo !== undefined ? foo : bar();
// !.
let a: string | null | undefined
a.length // error
a!.length // ok
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strictFunctionTypes
编译,函数参数类型是逆变的,父类 <= 子类,是错误。子类 <= 父类,是可以的。npm install @types/xxxx
可以配置tsconfig.json
文件中的declaration
和outDir
泛型用来来创建可重用的组件,一个组件可以支持多种类型的数据。这样用户就可以以自己的数据类型来使用组件。简单的说,“泛型就是把类型当成参数”。
用于删除修饰符
type A = {
a: string;
b: number;
}
type B = {
[K in keyof A]?: A[K]
}
type C = {
[K in keyof B]-?: B[K]
}
type D = {
readonly [K in keyof A]: A[K]
}
type E = {
-readonly [K in keyof A]: A[K]
}
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typescript的类型兼容是基于结构的,不是基于名义的。下面的代码在ts中是完全可以的,但在java等基于名义的语言则会抛错。
interface Named { name: string }
class Person {
name: string
}
let p: Named
// ok
p = new Person()
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const断言,typescript会为变量添加一个自身的字面量类型
对象字面量的属性,获得readonly的属性,成为只读属性
数组字面量成为readonly tuple只读元组
字面量类型不能被扩展(比如从hello类型到string类型)
// type '"hello"'
let x = "hello" as const
// type 'readonly [10, 20]'
let y = [10, 20] as const
// type '{ readonly text: "hello" }'
let z = { text: "hello" } as const
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unknown类型和any类型类似。与any类型不同的是。unknown类型可以接受任意类型赋值,但是unknown类型赋值给其他类型前,必须被断言
declare global {
interface Window {
myCustomFn: () => void;
}
}
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type A = UnionToIntersection<{a: string} | {b: string} | {c: string}>
// {a: string} & {b: string} & {c: string}
// 实现UnionToIntersection<T>
type UnionToIntersection<U> =
(U extends any ? (k: U) => void : never) extends ((k: infer I) => void) ? I : never
// https://stackoverflow.com/questions/50374908/transform-union-type-to-intersection-type
// https://jkchao.github.io/typescript-book-chinese/tips/infer.html#%E4%B8%80%E4%BA%9B%E7%94%A8%E4%BE%8B
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type A = ToNumber<'1'> // 1
type B = ToNumber<'40'> // 40
type C = ToNumber<'0'> // 0
// 实现ToNumber
type ToNumber<T extends string, R extends any[] = []> =
T extends `${R['length']}` ? R['length'] : ToNumber<T, [1, ...R]>;
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type A = Add<1, 2> // 3
type B = Add<0, 0> // 0
// 实现ADD
type NumberToArray<T, R extends any[]> = T extends R['length'] ? R : NumberToArray<T, [1, ...R]>
type Add<T, R> = [...NumberToArray<T, []>, ...NumberToArray<R, []>]['length']
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type A = SmallerThan<0, 1> // true
type B = SmallerThan<1, 0> // false
type C = SmallerThan<10, 9> // false
// 实现SmallerThan
type SmallerThan<N extends number, M extends number, L extends any[] = [], R extends any[] = []> =
N extends L['length'] ?
M extends R['length'] ? false : true
:
M extends R['length'] ? false : SmallerThan<N, M, [1, ...L], [1, ...R]>;
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type A = LargerThan<0, 1> // false
type B = LargerThan<1, 0> // true
type C = LargerThan<10, 9> // true
// 实现LargerThan
type LargerThan<N extends number, M extends number, L extends any[] = [], R extends any[] = []> =
N extends L['length'] ?
false : M extends R['length'] ?
true : LargerThan<N, M, [1, ...L], [1, ...R]>;
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type A = IsAny<string> // false
type B = IsAny<any> // true
type C = IsAny<unknown> // false
type D = IsAny<never> // false
// 实现IsAny
type IsAny<T> = true extends (T extends never ? true : false) ?
false extends (T extends never ? true : false) ?
true
:
false
:
false;
// 更简单的实现
type IsAny<T> = 0 extends (T & 1) ? true : false;
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type A = Filter<[1,'BFE', 2, true, 'dev'], number> // [1, 2]
type B = Filter<[1,'BFE', 2, true, 'dev'], string> // ['BFE', 'dev']
type C = Filter<[1,'BFE', 2, any, 'dev'], string> // ['BFE', any, 'dev']
// 实现Filter
type Filter<T extends any[], A, N extends any[] = []> =
T extends [infer P, ...infer Q] ?
0 extends (P & 1) ? Filter<Q, A, [...N, P]> :
P extends A ? Filter<Q, A, [...N, P]> : Filter<Q, A, N>
: N;
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type A = TupleToString<['a']> // 'a'
type B = TupleToString<['B', 'F', 'E']> // 'BFE'
type C = TupleToString<[]> // ''
// 实现TupleToString
type TupleToString<T extends any[], S extends string = '', A extends any[] = []> =
A['length'] extends T['length'] ? S : TupleToString<T, `${S}${T[A['length']]}`, [1, ...A]>
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type A = RepeatString<'a', 3> // 'aaa'
type B = RepeatString<'a', 0> // ''
// 实现RepeatString
type RepeatString<T extends string, C extends number, S extends string = '', A extends any[] = []> =
A['length'] extends C ? S : RepeatString<T, C, `${T}${S}`, [1, ...A]>
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type A = Push<[1,2,3], 4> // [1,2,3,4]
type B = Push<[1], 2> // [1, 2]
type C = Push<[], string> // [string]
// 实现Push
type Push<T extends any[], I> = T extends [...infer P] ? [...P, I] : [I]
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type A = Flat<[1,2,3]> // [1,2,3]
type B = Flat<[1,[2,3], [4,[5,[6]]]]> // [1,2,3,4,5,6]
type C = Flat<[]> // []
// 实现Flat
type Flat<T extends any[]> =
T extends [infer P, ...infer Q] ?
P extends any[] ? [...Flat<P>, ...Flat<Q>] : [P, ...Flat<Q>]
: [];
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type A = Shift<[1,2,3]> // [2,3]
type B = Shift<[1]> // []
type C = Shift<[]> // []
// 实现Shift
type Shift<T extends any[]> = T extends [infer P, ...infer Q] ? [...Q] : [];
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type A = Repeat<number, 3> // [number, number, number]
type B = Repeat<string, 2> // [string, string]
type C = Repeat<1, 1> // [1, 1]
type D = Repeat<0, 0> // []
// 实现Repeat
type Repeat<T, C, R extends any[] = []> =
R['length'] extends C ? R : Repeat<T, C, [...R, T]>
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type A = ReverseTuple<[string, number, boolean]> // [boolean, number, string]
type B = ReverseTuple<[1,2,3]> // [3,2,1]
type C = ReverseTuple<[]> // []
// 实现ReverseTuple
type ReverseTuple<T extends any[], A extends any[] = []> =
T extends [...infer Q, infer P] ?
A['length'] extends T['length'] ? A : ReverseTuple<Q, [...A, P]>
: A;
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type A = UnwrapPromise<Promise<string>> // string
type B = UnwrapPromise<Promise<null>> // null
type C = UnwrapPromise<null> // Error
// 实现UnwrapPromise
type UnwrapPromise<T> = T extends Promise<infer P> ? P : Error;
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type A = LengthOfString<'BFE.dev'> // 7
type B = LengthOfString<''> // 0
// 实现LengthOfString
type LengthOfString<T extends string, A extends any[] = []> =
T extends `${infer P}${infer Q}` ? LengthOfString<Q, [1, ...A]> : A['length']
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type A = StringToTuple<'BFE.dev'> // ['B', 'F', 'E', '.', 'd', 'e','v']
type B = StringToTuple<''> // []
// 实现
type StringToTuple<T extends string, A extends any[] = []> =
T extends `${infer K}${infer P}` ? StringToTuple<P, [...A, K]> : A;
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type A = LengthOfTuple<['B', 'F', 'E']> // 3
type B = LengthOfTuple<[]> // 0
// 实现
type LengthOfTuple<T extends any[], R extends any[] = []> =
R['length'] extends T['length'] ? R['length'] : LengthOfTuple<T, [...R, 1]>
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type A = LastItem<[string, number, boolean]> // boolean
type B = LastItem<['B', 'F', 'E']> // 'E'
type C = LastItem<[]> // never
// 实现LastItem
type LastItem<T> = T extends [...infer P, infer Q] ? Q : never;
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type A = FirstItem<[string, number, boolean]> // string
type B = FirstItem<['B', 'F', 'E']> // 'B'
// 实现FirstItem
type FirstItem<T> = T extends [infer P, ...infer Q] ? P : never;
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type A = FirstChar<'BFE'> // 'B'
type B = FirstChar<'dev'> // 'd'
type C = FirstChar<''> // never
// 实现FirstChar
type FirstChar<T> = T extends `${infer P}${infer Q}` ? P : never;
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type Foo = {
a: string
b: number
c: boolean
}
type A = MyPick<Foo, 'a' | 'b'> // {a: string, b: number}
type B = MyPick<Foo, 'c'> // {c: boolean}
type C = MyPick<Foo, 'd'> // Error
// 实现MyPick<T, K>
type MyPick<T, K extends keyof T> = {
[Key in K]: T[Key]
}
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type Foo = {
a: string
}
const a:Foo = {
a: 'BFE.dev',
}
a.a = 'bigfrontend.dev'
// OK
const b:MyReadonly<Foo> = {
a: 'BFE.dev'
}
b.a = 'bigfrontend.dev'
// Error
// 实现MyReadonly
type MyReadonly<T> = {
readonly [K in keyof T]: T[K]
}
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type Key = 'a' | 'b' | 'c'
const a: Record<Key, string> = {
a: 'BFE.dev',
b: 'BFE.dev',
c: 'BFE.dev'
}
a.a = 'bigfrontend.dev' // OK
a.b = 123 // Error
a.d = 'BFE.dev' // Error
type Foo = MyRecord<{a: string}, string> // Error
// 实现MyRecord
type MyRecord<K extends number | string | symbol, V> = {
[Key in K]: V
}
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type Foo = 'a' | 'b' | 'c'
type A = MyExclude<Foo, 'a'> // 'b' | 'c'
type B = MyExclude<Foo, 'c'> // 'a' | 'b
type C = MyExclude<Foo, 'c' | 'd'> // 'a' | 'b'
type D = MyExclude<Foo, 'a' | 'b' | 'c'> // never
// 实现 MyExclude<T, K>
type MyExclude<T, K> = T extends K ? never : T;
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type Foo = 'a' | 'b' | 'c'
type A = MyExtract<Foo, 'a'> // 'a'
type B = MyExtract<Foo, 'a' | 'b'> // 'a' | 'b'
type C = MyExtract<Foo, 'b' | 'c' | 'd' | 'e'> // 'b' | 'c'
type D = MyExtract<Foo, never> // never
// 实现MyExtract<T, U>
type MyExtract<T, U> = T extends U ? T : never
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type Foo = {
a: string
b: number
c: boolean
}
type A = MyOmit<Foo, 'a' | 'b'> // {c: boolean}
type B = MyOmit<Foo, 'c'> // {a: string, b: number}
type C = MyOmit<Foo, 'c' | 'd'> // {a: string, b: number}
// 实现MyOmit
type MyOmit<T, K extends number | string | symbol> = {
[Key in Exclude<keyof T, K>]: T[Key]
}
type MyOmit<T, K extends number | string | symbol> = Pick<T, Exclude<keyof T, K>>
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type Foo = 'a' | 'b' | null | undefined
type A = MyNonNullable<Foo> // 'a' | 'b'
// 实现NonNullable
type MyNonNullable<T> = T extends null | undefined ? never : T;
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type Foo = (a: string, b: number, c: boolean) => string
type A = MyParameters<Foo> // [a:string, b: number, c:boolean]
type B = A[0] // string
type C = MyParameters<{a: string}> // Error
// 实现MyParameters<T>
type MyParameters<T extends (...params: any[]) => any> =
T extends (...params: [...infer P]) => any ? P : never
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class Foo {
constructor (a: string, b: number, c: boolean) {}
}
type C = MyConstructorParameters<typeof Foo>
// [a: string, b: number, c: boolean]
// 实现MyConstructorParameters<T>
type MyConstructorParameters<T extends new (...params: any[]) => any> =
T extends new (...params: [...infer P]) => any ? P : never
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type Foo = () => {a: string}
type A = MyReturnType<Foo> // {a: string}
// 实现MyReturnType<T>
type MyReturnType<T extends (...params: any[]) => any> =
T extends (...params: any[]) => infer P ? P : never;
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class Foo {}
type A = MyInstanceType<typeof Foo> // Foo
type B = MyInstanceType<() => string> // Error
// 实现MyInstanceType<T>
type MyInstanceType<T extends new (...params: any[]) => any> =
T extends new (...params: any[]) => infer P ? P : never;
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function Foo(this: {a: string}) {}
function Bar() {}
type A = MyThisParameterType<typeof Foo> // {a: string}
type B = MyThisParameterType<typeof Bar> // unknown
// 实现MyThisParameterType<T>
type MyThisParameterType<T extends (this: any, ...params: any[]) => any> =
T extends (this: infer P, ...params: any[]) => any ? P : unknown;
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type Foo = [string, number, boolean]
type Bar = TupleToUnion<Foo> // string | number | boolean
// 实现TupleToUnion<T>
type TupleToUnion<T extends any[], R = T[0]> =
T extends [infer P, ...infer Q] ? TupleToUnion<Q, R | P> : R;
// 其他回答
type TupleToUnion<T extends any[]> = T[number]
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type Foo = {
a: string
b: number
c: boolean
}
// below are all valid
const a: MyPartial<Foo> = {}
const b: MyPartial<Foo> = {
a: 'BFE.dev'
}
const c: MyPartial<Foo> = {
b: 123
}
const d: MyPartial<Foo> = {
b: 123,
c: true
}
const e: MyPartial<Foo> = {
a: 'BFE.dev',
b: 123,
c: true
}
// 实现MyPartial<T>
type MyPartial<T> = {
[K in keyof T]?: T[K]
}
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// all properties are optional
type Foo = {
a?: string
b?: number
c?: boolean
}
const a: MyRequired<Foo> = {}
// Error
const b: MyRequired<Foo> = {
a: 'BFE.dev'
}
// Error
const c: MyRequired<Foo> = {
b: 123
}
// Error
const d: MyRequired<Foo> = {
b: 123,
c: true
}
// Error
const e: MyRequired<Foo> = {
a: 'BFE.dev',
b: 123,
c: true
}
// valid
// 实现MyRequired<T>
type MyRequired<T> = {
[K in keyof T]-?: T[K]
}
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type A = LastChar<'BFE'> // 'E'
type B = LastChar<'dev'> // 'v'
type C = LastChar<''> // never
// 实现FirstChar<T>
type LastChar<T extends string, A extends string[] = []> =
T extends `${infer P}${infer Q}` ? LastChar<Q, [...A, P]> :
A extends [...infer L, infer R] ? R : never
;
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// https://stackoverflow.com/questions/53984650/typescript-never-type-inconsistently-matched-in-conditional-type
// https://www.typescriptlang.org/docs/handbook/advanced-types.html#v
type A = IsNever<never> // true
type B = IsNever<string> // false
type C = IsNever<undefined> // false
// 实现IsNever<T>
type IsNever<T> = [T] extends [never] ? true : false;
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type A = KeyToUnion<{
a: string;
b: number;
c: symbol;
}>
// 'a' | 'b' | 'c'
// 实现KeyToUnion
type KeyToUnion<T> = {
[K in keyof T]: K;
}[keyof T]
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type A = ValuesToUnion<{
a: string;
b: number;
c: symbol;
}>
// string | number | symbol
// ValuesToUnion
type ValuesToUnion<T> = T[keyof T]
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bigfrontend.dev/zh/typescri…[2]
type IsAny<T> = 0 extends (T & 1) ? true : false;
type IsNever<T> = [T] extends [never] ? true : false;
type TwoAny<A, B> = IsAny<A> extends IsAny<B> ? IsAny<A> : false;
type TwoNever<A, B> = IsNever<A> extends IsNever<B> ? IsNever<A> : false;
type SingleAny<A, B> = IsAny<A> extends true ? true : IsAny<B>
type SingleNever<A, B> = IsNever<A> extends true ? true : IsNever<B>
type FindIndex<T extends any[], E, A extends any[] = []> =
T extends [infer P, ...infer Q] ?
TwoAny<P, E> extends true ?
A['length']
:
TwoNever<P, E> extends true ?
A['length']
:
SingleAny<P, E> extends true ?
FindIndex<Q, E, [1, ...A]>
:
SingleNever<P, E> extends true ?
FindIndex<Q, E, [1, ...A]>
:
P extends E ? A['length'] : FindIndex<Q, E, [1, ...A]>
:
never
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type A = Trim<' BFE.dev'> // 'BFE'
type B = Trim<' BFE. dev '> // 'BFE. dev'
type C = Trim<' BFE . dev '> // 'BFE . dev'
type StringToTuple<T extends string, A extends any[] = []> =
T extends `${infer K}${infer P}` ? StringToTuple<P, [...A, K]> : A;
type TupleToString<T extends any[], S extends string = '', A extends any[] = []> =
A['length'] extends T['length'] ? S : TupleToString<T, `${S}${T[A['length']]}`, [1, ...A]>
type Trim<T extends string, A extends any[] = StringToTuple<T>> =
A extends [infer P, ...infer Q] ?
P extends ' ' ?
Trim<T, Q>
:
A extends [...infer M, infer N] ?
N extends ' ' ?
Trim<T, M>
:
TupleToString<A>
:
''
:
'';
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还有更多 UnionToTuple
, IntersectionToUnion
?
关于本文
https://juejin.cn/post/6988763249982308382