// 实现 Assign （类似js中Object.assign(A,B)效果，存在B与A存在相同属性时，B会覆盖A中对应的属性值) type MergeArrayTarget<A,B> = { [key in keyof B as key extends number ? key : never]: B[key] } & { [key in keyof A as key extends number ? never : key]: A[key] } type MergeTupleTarget<A,B> = { [key in keyof B as key extends `${number}` ? key : never]: B[key] } & { [key in keyof A as key extends (`${number}` | number) ? never : key]: A[key] }; type MergeTarget<T> = { [K in keyof T]: T[K] }; type NormalMerge<A,B> = // A & B 共有属性 { [key in Extract<keyof B, keyof A>]: B[key] } & // A 中去除 B 上相同的属性 { [key in keyof A as key extends Extract<keyof B, keyof A> ? never : key]: A[key] } & // B 中去除 A 上相同的 string | number 类型属性 { [key in Exclude<keyof B, keyof A> as key extends string | number ? key : never]: B[key] } type Assign<A, B> = AnyOrNever<A> extends true ? A : // current A is any or never [A] extends [LiteralType] ? A : // current A IsLiteral : [A] extends [object] ? // charge A is interface AnyOrNever<B> extends true ? A : [B] extends [LiteralType] ? A : [B] extends [object] ? IsArray<B> extends true ? MergeTarget<MergeArrayTarget<A, B>> : IsTuple<B> extends true ? MergeTarget<MergeTupleTarget<A, B>> : MergeTarget<NormalMerge<A,B>> : A : ([A] extends [symbol] ? symbol : // symbol will return it self never); // void or undefined (throw error in js)