What "!" after variable in JavaScript/TypeScript means?

Question

Let's have this piece of code written in TypeScript:

try {
    // To call async functions, use redux-saga's `call()`.
    const res = yield call(callApi, 'get', API_ENDPOINT, '/heroes')

    if (res.error) {
      yield put(fetchError(res.error))
    } else {
      yield put(fetchSuccess(res))
    }
  } catch (err) {
    if (err instanceof Error) {
      yield put(fetchError(err.stack!)) // <----------------------------??
    } else {
      yield put(fetchError('An unknown error occured.'))
    }
  }

What does err.stack! in code mean? I am not familiar with this from JavaScript.