Array of only strings or only numbers

I can't seem to figure out how to work with an array that will be made of only strings or only numbers, basically something with type string[] | number[]. On such an array I can't seem to call .push() on it because the type for the arguments to .push() will be never.

My use case is this:

I have an interface like this

export interface Response {
  numbers: Array<number>;
  strings: Array<string>;
}

export type ResponseKey = keyof Response;

Then later if I index into a Response object

const response: Response = { numbers: [], strings: [] };
let key: ResponseKey;
// dynamicaly assign the key somehow ...
const value = response[key];

Now the type for value is string[] | number[].

TypeScript infers every item type to string & number which gives never because these types have nothing in common.

You could split your code into two if statements to narrow the key to its actual type:

export interface Response {
    numbers: Array<number>;
    strings: Array<string>;
}

type ResponseKey = keyof Response;


const response: Response = { numbers: [], strings: [] };
declare let key: ResponseKey;
// dynamicaly assign the key somehow ...


if (key === 'numbers') {
    const value = response[key]; // number[]
    value.push(2);
}

if (key === 'strings') {
    const value = response[key]; // string[]
    value.push('test');
}

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