JavaScript · Lesson 15 of 15

Testing JavaScript

Unit tests with Node's built-in runner or Jest, assertions and mocking.

Advanced
13 min read
3 objectives

Before this lessonLesson 14: Iterators, Generators, Map and Set

What you will learn

Write unit tests
Run tests from npm
Mock a dependency

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Tests are code that checks your code. They catch regressions when you change things later and document how functions should behave.

The built-in Node test runner

// sum.js
export function sum(a, b) { return a + b; }

// sum.test.js
import { test } from "node:test";
import assert from "node:assert/strict";
import { sum } from "./sum.js";

test("adds numbers", () => {
  assert.equal(sum(2, 3), 5);
});

test("adds negatives", () => {
  assert.equal(sum(-2, -3), -5);
});

node --test

Jest / Vitest style

Most projects use Jest or Vitest. Their API is nearly identical: describe groups tests, test defines one, and expect checks a value.

import { describe, test, expect } from "vitest";
import { sum } from "./sum.js";

describe("sum", () => {
  test("adds", () => {
    expect(sum(1, 2)).toBe(3);
  });
  test("arrays compare by value with toEqual", () => {
    expect([1, 2]).toEqual([1, 2]);
  });
  test("throws", () => {
    expect(() => JSON.parse("{")).toThrow();
  });
});

Async tests and mocks

import { test, expect, vi } from "vitest";

test("loads a user", async () => {
  const fetchUser = vi.fn().mockResolvedValue({ name: "Ada" });   // fake dependency
  const user = await fetchUser(1);
  expect(user.name).toBe("Ada");
  expect(fetchUser).toHaveBeenCalledWith(1);
});

Add it to npm

npm pkg set scripts.test="vitest run"
npm test

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