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type

Omit & Pick​

Omit = except, Pick = include

extends​

继承的对象一定有指定对象的属性

Arrays​

Note that [number] is a different thing; refer to the section on Tuples.

type a = [1,2,3];
type b = a[number]; // 是union，1 | 2 | 3

keyof​

type TupleToObject<T extends readonly any[]> = {
  [P in K]: P; // 是index索引，0123
}
type MyPick<T, K extends keyof T> = {
   [P in K]: T[P];
}

T = ['title', 'completed', 'invalid'] K 是 T 的其中一个 key，比如 'invalid'

readonly​

infer & spread​

infer 假设， spread 展开...

type a = [1,2,3]
type b = a extends [infer First, ...infer rest] ? First : never // b = 1

as const​

as const = readonly

const a = [1,2,3] as const
const b = [1,2,3]
type a1 = typeof a // readonly [1, 2, 3]
type b1 = typeof b // number[]

PromiseLike​

Strict Null Checks​

严格判断 Null 是否为 true