TypeScript keyof和Indexer兼容性？

Question

I have a method signature which expect a second argument of keyof Array<...> . Since Array interface defines an indexer [n: number]: T; I'd expect to be some way to refer to some index in the method.

But I can't find how. I tried the following:

MyMethod(myArray, 0); // Argument of type '0' is not assignable to parameter of type ...
MyMethod(myArray, [0]); // Argument of type 'number[]' is not assignable to parameter of type ...
MyMethod(myArray, '0'); // Argument of type '"0"' is not assignable to parameter of type ...
MyMethod(myArray, '[0]'); //  Argument of type '"[0]"' is not assignable to parameter of type ...

And none is working.

Answer 1

You could always try:

function myFun<T>(arr: Array<T>, index: number) {
  return arr[index];
}

keyof Array<...> refers to all property names of arrays, like length , toString , push , pop , etc. Number indices aren't part of the Array interface in the same manner because they're a lookup type :

interface Array<T> {
  length: number;
  toString(): string;

   ... etc ...

  [n: number]: T;
}

Consider a simpler interface:

interface Arr<T> {
  [n: number]: T;
}

const variable: keyof Arr<number>; // variable is of type "never"

This is actually a shortcoming of the language. See also this issue on GitHub :

We can not list all these numeric string literals (effectively). keyof Container<T> would be number | numericString number | numericString with numericString as type of all numeric string literals with a corresponding number literal. number | string number | string wouldn't be correct because not every string is a numericString . Also not every string sequence of digit characters is a numbericString since number has a maximum and minimum value.