TypeScript 常见问题

几个关键点

• TypeScript是结构化类型（结构相同那就是一样的类型）

• 子类型可以赋值给父类型(animal = dog)，反之不可

• 属性多的是子类型，但是联合类型越多反而是父类型

• 用是否安全来考虑类型是否兼容的问题

协变、逆变（啥玩意儿？）

忘记那些名词，以类型安全来考虑就行。

简单类型

• 简单类型1

type Parent = {
    name: string
}

type Child = {
    name: string
    age: number
}

let p: Parent = { name: 'parent' }
let c: Child = { name: 'child', age: 1 }

p = c  // ok
c = p  // error: Property 'age' is missing in type 'Parent' but required in type 'Child'

• 简单类型2（联合类型）

type One = string
type Two = string | number
type Three = string | number | boolean

let one:One = ''
let two:Two = 2
let three:Three = false

one = two    // error，如你所见，two 可能会是 number，所以类型不安全
one = three  // error，类似上理

two = one    // ok
two = three  // error，类似上理

three = one  // ok
three = two  // ok

• 复合类型

type Parent = {
    name: string
}

type Child = {
    name: string
    age: number
}

type Comp<T> = T[]


let p: Parent = { name: 'parent' }
let c: Child = { name: 'child', age: 1 }
let cp: Comp<Parent> = [p]
let cc: Comp<Child> = [c]

p = c  // ok
c = p  // error: missing 'age'

cp = cc  // ok
cc = cp  // error: Type 'Parent' is not assignable to type 'Child'

cp = [c]  // ok
cc = [p]  // error: Type 'Parent' is not assignable to type 'Child'

• 函数

type B = (a: A | A[]) => void
const b1: B = (a: A) => { }    // error: Type 'string[]' is not assignable to type 'string'
const b2: B = (a: A[]) => { }  // error: Type 'string' is not assignable to type 'string[]'

type Parent = {
    name: string
}

type Child = {
    name: string
    age: number
}

type FuncParent = (p: Parent) => void
type FuncChild = (c: Child) => void


let p: Parent = { name: 'parent' }
let c: Child = { name: 'child', age: 1 }

let fp: FuncParent = (p) => { }
let fc: FuncChild = (c) => { }

p = c  // ok
c = p  // error: Property 'age' is missing in type 'Parent' but required in type 'Child'

fp = fc  // error: Type 'Parent' is not assignable to type 'Child'
fc = fp  // ok

本节参考

1. README - TypeScript Deep Dive (gitbook.io)

2. 快速理解 TypeScript 的逆变、协变、双向协变、不变 - 知乎 (zhihu.com)