Instanceof type guard can't narrow the type of a variable used inside a callback

Issue

const userAPI = {
  async getById() { }
}

const batchedUpdates = (callback: () => any) => {
  callback();
}
const setError = (error: Error | null) => { }

async function main() {
  const result: { error: Error | null, data: any | null } = {
    error: null,
    data: null
  }
  try {
    const user = await userAPI.getById();
  } catch (error) {
    if (error instanceof Error) {
      result.error = error;
      batchedUpdates(() => {
        setError(error);
      })
    }
  }
}

Got error at the line of setError(error) statement:

Argument of type 'unknown' is not assignable to parameter of type 'Error | null'.(2345)

I don't want to repeat the instanceof type guard in the callback of batchedUpdates, what's the right approach to handle this situation? Thanks.

TypeScript Playground

Solution

Control Flow Analysis does not cross callback boundaries. This is explained in the canonical issue Trade-offs in Control Flow Analysis

A workaround is to use a local variable to freeze the type:

const userAPI = {
  async getById() { }
}

const batchedUpdates = (callback: () => any) => {
  callback();
}
const setError = (error: Error | null) => { }

async function main() {
  const result: { error: Error | null, data: any | null } = {
    error: null,
    data: null
  }
  try {
    const user = await userAPI.getById();
  } catch (error) {
    if (error instanceof Error) {
      result.error = error;
      const localError = error
      batchedUpdates(() => {
        setError(localError);
      })
    }
  }
}

Playground Link

Answered By - Titian Cernicova-Dragomir