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TypeScript key and index compatibility?

I have a method signature that expects a second argument keyof Array<...>

. Since the interface Array

defines an index [n: number]: T;

, I would expect some way to refer to some index in a method.

But I cannot find how to do this. 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 nobody works.

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1 answer


You can always try:

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

keyof Array<...>

It refers to all the property names arrays, such as length

, toString

, push

, pop

etc. Number indices are not part of the interface Array

in the same way, they are a search 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 flaw in the language. See also this issue on GitHub :

We cannot list all these numeric string literals (efficiently). keyof Container<T>

will be number | numericString

c numericString

as the type of all numeric string literals with the corresponding number literal. number | string

will not be correct because not everyone string

is numericString

. In addition, not every string is a character digit sequence numbericString

because a number has a maximum and minimum value.

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