Last updated: 25.02.2024

Context:

• Environment: ESM, Node.js 20, TypeScript project.
• Dependencies:
  • Mocha version: "mocha": "10.3.0"
  • Chai version: "chai": "4.4.1"

Problem:

Consider a service attempting to create a new database resource. If initial business validation fails, an error is thrown. To test this behavior, you might write:

it('should raise a validation error when title is too short', async () => {
  const service = taskService()

  expect(service.createNewTask({ title: ' FV' })).to.throw(
    'Validation failed, double check your form please and try again'
  )
})

Test will fail with following error:

1) task service
       should raise a validation error when title is too short:
     AssertionError: expected Promise{…} to be a function
      at Context.<anonymous> (src/tasks/services/task-service.spec.ts:46:54)
      at process.processImmediate (node:internal/timers:478:21)

Root cause:

The test incorrectly attempts to use expect().to.throw with a promise returned by an async function, which is not supported by Chai's expect directly.

Solution:

• install chai-as-promised and use it in your test

import { expect, use } from "chai";
import chaiAsPromised from "chai-as-promised";

import { taskService } from "./task-service.js";

use(chaiAsPromised);

Switch assertion to to.be.rejectedWith and await the assertion

it("should raise a validation error when title is too short", async () => {
		const service = taskService();

		await expect(service.createNewTask({ title: " FV" })).to.be.rejectedWith(
			"Validation failed, please double check your form and try again"
		);
	});

You can read more about chai as promised: here

There is also Github issue about native support for promise in Chai: here

Alternative:

You could also leverage the approach without chai-as-promised simply by wrapping the test in try catch block

it("should raise a validation error when title is too short", async () => {
		const service = taskService();

		try {
			await service.createNewTask({ title: " FV" });
		} catch (err) {
			// Note: error is unknown here, use type-guard or ts-ignore
			expect(err.message).to.equal(
				"Validation failed, please double check your form and try again"
			);
		}
});