In Typescript "keyof Record<string, string>" is not "string"

Issue

I'm hitting this wall since days now and I'm not sure anymore if I'm the problem or if typescript is broken...

I defined a generic class with the generic extending Record<string, string>:

class DbTable<ColumnDefinitions extends Record<string, string>> { /* ... */ }

At a later point I want to use the exact keys (e.g. 'id' | 'name' instead of just string), so I use the keyof operator on the generic, like this:

function selectFields(fields: (keyof ColumnDefinitions)[]) { /* ... * / }

But keyof ColumnDefinitions resolves to string | number | symbol instead of just being of type string.

What did I miss?!

Here is a TS playground link with an example.

Solution

You can extract only strings from string | number | symbol in the method signature.

Instead of

fields: (keyof T)[]

you can write

fields: (Extract<keyof T, string>)[]

and the error goes away.

TypeScript Playground

Answered By - Lazar Ljubenović