要求

从某个联合类型中，筛选出和另外一个联合类型相交的部分。

// your code 
// type YourTypeFunc...

type Union1 = "A" | "B" | "C";
type Union2 = "A" | "B";

// result 
type res = YourTypeFunc<Union1, Union2> // "A" | "B"

可参考的知识点

1. A conditional type T extends U ? X : Y is either resolved to X or Y, or deferred because the condition depends on one or more type variables.
2. Conditional types in which the checked type is a naked type parameter are called distributive conditional types. Distributive conditional types are automatically distributed over union types during instantiation. For example, an instantiation of T extends U ? X : Y with the type argument A | B | C for T is resolved as (A extends U ? X : Y) | (B extends U ? X : Y) | (C extends U ? X : Y).

知识链接

1. 范性
2. 条件类型
3. 类型分发

问题 & 解答

1. 答题入口：Extract
2. 解答

使用场景

// 判断两个对象的部分公有fields属性值是否相等


// @ts-ignore JS写法
function isEqual1(obj1, obj2, fields) {
// @ts-ignore
  return fields.every((field) => obj1[field] ===obj2[field]);
}

// 用到Extract的TS写法
function isEqual2<T extends Record<string, any>, K extends Record<string, any>>  (obj1: T, obj2: K,fields: Array<Extract<keyof T, keyof K>>
): boolean {
  return fields.every((field) => obj1[field] ===obj2[field]);
} 

isEqual1(
  { a: 1, b: 1, c: 1},
  { a: 1, b: 2 },
  ['c']
)

isEqual2(
  { a: 1, b: 1, c: 1},
  { a: 1, b: 2 },
  ['c']  // Type '"c"' is not assignable to type '"a" | "b"'.
)