expecting-errors.md

Expecting Errors

To assert that a test throws, add a then clause named expect_error. vitest-gwt (via gwt-runner) detects it, runs it, and passes the thrown error as the argument. Assert on the error and the test passes.

If code throws an error and there is no expect_error clause, the test fails with that error.

test("should throw error", {
  given: {
    an_error_message,
  },
  when: {
    throwing_error,
  },
  then: {
    expect_error,
  },
});

function an_error_message() {
  this.error_message = "an error";
}

function throwing_error() {
  throw new Error(this.error_message);
}

function expect_error(error) {
  expect(error.message).toBe(this.error_message);
}

Named expect_error

The key must be expect_error, but the function itself can have any name. Using a more descriptive function name keeps assertions readable:

test("rejects invalid email", {
  given: { invalid_email },
  when: { validating_email },
  then: {
    expect_error: rejected_with_server_message,
  },
});

function rejected_with_server_message(error) {
  expect(error).toBe("error message from server");
}

Scenarios

expect_error also works inside scenario then blocks. See Scenarios.

Next: Scenarios.