打字稿中相同實現的不同返回類型推斷

Question

我想了解為什么以下 2 個實現之間存在類型差異。 它們都生成相同的結果，但它們的返回類型各不相同。

const from = [
    () => console.log('first'), 
    () => console.log('second'), 
    () => console.log('third')];

const to = {
    first: 1,
    second: 2,
    third: 3,
};

const aggFn = (from: Function[], to: { [as: string]: any }): Record<keyof typeof to, Function> => {
    return ({
        first: from[0],
        second: from[1],
        third: from[2],
    } as unknown) as Record<keyof typeof to, Function>;
};

const agg = aggFn(from, to);

這里 agg 的返回類型是Record<"string" | "number", Function> , 至於第二個代碼

const from = [
    () => console.log('first'), 
    () => console.log('second'), 
    () => console.log('third')];

const to = {
    first: 1,
    second: 2,
    third: 3,
};

const aggFn2 = <T>(from: Function[], to: { [as: string]: any }): T => {
    return ({
        first: from[0],
        second: from[1],
        third: from[2],
    } as unknown) as T;
};

const agg2 = aggFn2<Record<keyof typeof to, Function>>(from, to);

返回類型是Record<"first" | “第二” | “第三”，功能>

Answer 1

不同之處在於to在兩個版本的typeof to引用不同的變量：在第一個片段中它引用類型為{ [as: string]: any }的局部變量，在另一個片段中它引用全局變量to . 該變量是通過分配給它的值鍵入的，該值的類型為{ first: string, second: string, third: string } 。

函數的實際類型應該是（這實際上符合它的行為，它允許打字稿根據to參數推斷通用參數）：

  <T> (to: Record<T, any>): Record<T, Function>