"Type 'string' cannot be used to index type 'T'" when using generics

Question

I'm using generics in TypeScript (v4.5.4) and the following code throws an error:

function foo<T extends Record<string, boolean>>(obj: T, key: string) {
    obj[key] = true; // ERROR: Type 'string' cannot be used to index type 'T'
}

Playground link

This makes no sense to me whatsoever, it's even more nonsensical given the fact that the following actually works with no errors:

const key: string = '';
const obj: Record<string, boolean> = {};
obj[key] = true; // NO ERROR

While as far I'm concerned the first code snippet is effectively doing the exact same thing.

I would like to know why this happens, and also how I could rectify the problem in the first snippet.

specify parameter key as type keyof T will get rid of your current error, but it also leads to another one (Type 'boolean' is not assignable to type 'T[keyof T]'.(2322))... I would just cast true as T[keyof T], @jcalz explains this pretty well in another answer here. — Zou Jeff
– Zou Jeff, Commented Mar 18, 2022 at 7:00

Dominik Chudy · Accepted Answer · 2022-03-18 06:34:11Z

1

As far as I know, this is a bug, which was discussed pretty deeply on GitHub. Here's the link: https://github.com/microsoft/TypeScript/issues/47357

answered Mar 18, 2022 at 6:34

Dominik Chudy

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