"TS2322: Type 'Timeout' is not assignable to type 'number'" when running unit tests

You could try with using window.setTimeout instead of just setTimeout, this way the typescript one will be explicitly used

You can use:

let timeoutId: null | ReturnType<typeof setTimeout> = null
...
timeoutId = setTimeout(...)

It'll pick correct declaration depending on your context.

I'm seeing this discrepency when using vscode/tsc (NodeJS.Timeout) and running ts-jest (number). This is the only way the whole thing typechecks on both sides.