snippet showing promise rejection in a test using try and catch

promise.test_1.ts

test("test foo", async () => {
  function createPromiseThatRejects(): Promise<void> {
    return new Promise<void>((_, rejectFn) => {
      setTimeout(() => {
        rejectFn(new Error("Rejected"))
      }, 1_000)
    })
  }

  expect(createPromiseThatRejects()).rejects.toThrow("Rejected")

  async function main(): Promise<string> {
    try {
      await createPromiseThatRejects()
      return "ok"
    } catch (error) {
      return "error"
    }
  }

  expect(await main()).not.toBe("ok")
  expect(await main()).toBe("error")

})

promise.test_2.ts

test("test if statement to reject or resolve", async () => {
  let toReject = true
  let reject_count = 0
  let resolve_count = 0

  function createPromise(): Promise<string> {
    return new Promise<string>((resolveFn, rejectFn) => {
      setTimeout(() => {
        if (toReject) {
          reject_count = reject_count + 1
          rejectFn(new Error("Rejected")) // 😮 This does not throw error here!
        } else {
          resolve_count = resolve_count + 1
          resolveFn("ok")
        }
      }, 1_000)
    })
  }

  expect(reject_count).toBe(0)
  expect(resolve_count).toBe(0)

  const p_1 = createPromise()
  try {
    const _result = await p_1 // 💣 throws Error!
    expect(_result).toBe("ok")
  }
  catch (e: unknown) {
    let error = e as Error
    expect(error).toBeInstanceOf(Error)
    expect(error.message).toBe("Rejected")
  }

  expect(reject_count).toBe(1)
  expect(resolve_count).toBe(0)

  toReject = false

  const p_2 = createPromise()
  try {
    const _result = await p_2 // 💣 may throw Error!
    expect(_result).toBe("ok")
  } finally { }

  expect(reject_count).toBe(1)
  expect(resolve_count).toBe(1)

})