TypeScript 泛型 TypedArray : TypedArray is not assignable to T

Question

我正在学习泛型，并用编译器解决了这个问题:

type FloatArray = Float32Array | Float64Array;
type IntegerArray =
  | Int8Array
  | Uint8Array
  | Int16Array
  | Uint16Array
  | Int32Array
  | Uint32Array
  | Uint8ClampedArray;
type TypedArray = FloatArray | IntegerArray;

export function identityArray<T extends TypedArray>(array: T): T {
  return array.subarray(0);
}
// Type 'TypedArray' is not assignable to type 'T'

我在这里做错了什么？

Answer 1

没有类型断言的替代解决方案:

type TypedArray = FloatArray | IntegerArray;
type WithSubArray<T extends TypedArray> = { subarray(begin?: number, end?: number): T };

export function identityArray<T extends TypedArray>(array: WithSubArray<T>): T {
    return array.subarray(0);
}

const res1 = identityArray(new Int8Array(2)) // Int8Array 👌
const res2 = identityArray(new Float32Array(2)) // Float32Array 👌
const res3 = identityArray([1, 2, 3]) // ✖

如 T声明为函数返回类型，确保也返回 T正好在函数体中。通过使用 WithSubArray我们可以让编译器清楚， array.subarray返回 T而不是 TypedArray .

TS Playground to try it out