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

I have two TypeScript packages, and one package (Package A) depends on the other (Package B). Each package has a unit test set up using Karma. When I run unit tests for each individually after installing all dependencies from NPM, the unit tests run fine. However, if I use npm link package-b in Package A and run Package A's unit tests then, I get the error stated in the title: "TS2322: Type 'Timeout' is not assignable to type 'number'."

The line in question is a call to setTimeout. After digging, I found that while running the tests separately without npm link, TypeScript correctly identifies the setTimeout signature in typescript/lib/lib.dom as the desired type, but in the failing case after using npm link it is using using Node's setTimeout signature in @types/node/index. I confirmed this by changing the return type on setTimeout to string and observing the same error with string in the place of Timeout.

What I am not certain of is why the TypeScript compiler has decided to use the alternative definition in this specific case, nor how I can convince it to use the desired definition. I am happy to post some code, but I am not sure what would be useful in this case given all that is on the failing line is the setTimeout call.

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.

By default, typescript includes all ./node_modules/@types/*. If you have ./node_modules/@types/node there, its timeout typings will override the web typing (that returns a number, and not a NodeJS.Timeout).

You can remedy to that by explicitly emptying your types in your tsconfig.json:

{
  "compilerOptions": {
    "types": []
  }
}

Realistically you're probably in a project where you need other types and libs so you might wanna bring back ES and DOM libs:

{
  "compilerOptions": {
    "types": [],
    "lib": ["ESNext", "DOM"]
  }
}

You can use something like:

let myTimeOut: NodeJS.Timeout | null = null;

myTimeOut = setTimeout(...);

Then when you reset your variable to initial status, you can simply:

myTimeOut = null;