Typescript Array[T] 接受 {..T} 作为有效类型

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问题描述

当我使用 useState<Options[]> 作为反应钩子时，当我错误地输入 {...options} 而不是 [ 时，这让我感到困惑...options]，我只是在尝试访问数组"(对象)的 .map 并得到 TypeError 时才发现.让我举个例子来说明我的问题:

This bit me when using useState<Options[]> as a react hook, when I mistyped {...options} instead of [...options], I only found out when I tried to access .map of "array" (object) and got a TypeError. Let me give you an example that can clarify my question:

  interface Option {
    title: string;
    isSelected?: boolean;
  }

  function myFunc(options: Option[]): void {}

  const options: Option[] = [{title: 'Male', isSelected: true}, {title: 'Female'}, {title: 'Other'}];
  const options2 = {...options};
  myFunc(options);
  myFunc(options2); // why isn't this an error ?

推荐答案

这似乎确实是 Typescript 本身的问题:

This appears to be indeed a problem with Typescript itself:

let c: number[] = { ...[0, 1, 2] }; // compiles
c.fill(0) // runtime error

let d: number[] = Object.assign({}, [0, 1, 2]);  // compiles
d.fill(0) // runtime error

此外，这也会在运行时编译和中断:

Moreover, this also compiles and breaks at run-time:

class E {
    method() { }
}

let e: E = Object.assign({}, new E)
e.method() // runtime error

我猜这是因为 Object.assign 被声明为

I guess this is because Object.assign is declared as

assign<T, U>(target: T, source: U): T & U;

这是不正确的，因为 assign 返回的内容实际上并没有扩展 U.assign 的返回类型应该类似于 T &OwnProperties，但目前无法实现，请参阅

which is incorrect, since what assign returns doesn't actually extend U. The return type of assign should be something like T & OwnProperties<U>, but this is currently not possible, see

https://github.com/microsoft/TypeScript/issues/28801
https://github.com/microsoft/TypeScript/issues/9726

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