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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →To test a Node.js application with Mocha, install Mocha locally, put a test in test/, write an assertion, and run npx mocha. This tutorial walks through that setup, then shows how to test asynchronous code, isolate tests with hooks, choose CommonJS or ESM, and keep settings repeatable.
Check Node.js and install Mocha
Mocha’s getting-started guide documents its v12.0.0 requirement as Node.js ^20.19.0 || >=22.12.0. Check the runtime in your project before installing:
node --versionIf your version does not meet that requirement, use a compatible Node.js release or consult the documentation for a Mocha version that supports your runtime. Install Mocha as a development dependency so the project records the test runner:
npm install --save-dev mochaThe equivalent commands for other package managers are pnpm add --save-dev mocha and yarn add --dev mocha. These are installation alternatives; the test workflow below remains the same. See Mocha’s Getting Started guide for current requirements and package-manager guidance.
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Write and run your first test
Mocha discovers tests in the conventional test/ directory. This CommonJS-compatible example uses Node’s built-in assertion module, so it needs no assertion-library dependency. Save it as test/example.test.js:
const assert = require('node:assert');
describe('Array#indexOf()', function () {
it('returns -1 when the value is absent', function () {
assert.strictEqual([1, 2, 3].indexOf(4), -1);
});
});describe groups related behavior and it states the expected outcome. Run the suite from the project root:
npx mochaThe getting-started guide illustrates a successful run with 1 passing; that is an example of expected output, not a result from an independently run test here. If the command cannot find a test, verify the working directory and that the file is inside test/.
To make the command available through the project’s usual package script, add this entry to package.json:
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{
"scripts": {
"test": "mocha"
}
}Then run npm test. The script is simply a repeatable wrapper around Mocha’s command.
Test your application’s behavior
A useful test exercises an application behavior rather than merely confirming that a library behaves as documented. For example, if a project exports a function named normalizeName, a test could assert the contract the application expects:
const assert = require('node:assert');
const { normalizeName } = require('../src/normalize-name');
describe('normalizeName', function () {
it('trims surrounding whitespace', function () {
assert.strictEqual(normalizeName(' Ada '), 'Ada');
});
});This assumes your project actually exports that function from src/normalize-name.js; change the import and expected behavior to match your code. Keep each test focused on a behavior and an observable result. Use strict assertions such as assert.strictEqual where exact equality is the contract, and choose other assertions from Node’s node:assert API when the behavior calls for them.
Choose one completion pattern for asynchronous tests
Mocha needs to know when an asynchronous test has finished. Choose the completion mechanism that matches the API under test: callback completion for callback APIs, a returned Promise for Promise APIs, or async/await for an asynchronous flow. Do not return a Promise and also call done() in the same test; Mocha treats the two signals as overspecified completion. The same patterns apply to asynchronous hooks. See Mocha’s asynchronous-code guide.
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Callback API: signal completion with done
it('loads a record through a callback API', function (done) {
loadRecord('record-1', function (err, record) {
if (err) return done(err);
try {
assert.strictEqual(record.id, 'record-1');
done();
} catch (assertionError) {
done(assertionError);
}
});
});This is an illustrative pattern: replace loadRecord with your callback-based function. Pass callback errors to done(err). Assertions inside callbacks should also reach Mocha as errors, as shown, rather than being lost in asynchronous code.
Promise API: return the Promise
it('resolves with a record', function () {
return loadRecord('record-1').then(function (record) {
assert.strictEqual(record.id, 'record-1');
});
});Returning the Promise lets Mocha wait for settlement and report a rejection as a failure. The function name is illustrative; use your Promise-returning API.
Async function: await the operation
it('resolves with a record', async function () {
const record = await loadRecord('record-1');
assert.strictEqual(record.id, 'record-1');
});An async test returns a Promise automatically. Use either this pattern or the explicit Promise-returning form, not done as well.
Use hooks to set up and clean up fixtures
The default BDD interface provides four hooks. Use suite-level hooks for work that genuinely happens once around a group of tests, and per-test hooks when each case needs an independent setup and cleanup:
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| Hook | When it runs | Typical role |
|---|---|---|
before |
Once before tests in its suite | Create a shared fixture or establish suite prerequisites |
after |
Once after tests in its suite | Release suite-level resources |
beforeEach |
Before each test in its suite | Reset state so a case starts independently |
afterEach |
After each test in its suite | Restore state or clean up per-test resources |
Hooks can be synchronous or asynchronous. This illustrative example uses a hypothetical fixture API:
describe('user service', function () {
let fixture;
beforeEach(async function () {
fixture = await createUserFixture();
});
afterEach(async function () {
await fixture.dispose();
});
it('finds a user by ID', async function () {
const user = await fixture.service.findById(fixture.user.id);
assert.strictEqual(user.id, fixture.user.id);
});
});createUserFixture and its methods are examples, not an included database implementation. Prefer local hooks within the suite that needs them; this makes dependencies and cleanup easier to see. For hooks that apply at the root level, Mocha recommends Root Hook Plugins rather than relying on root hooks since Mocha v8. Details are in the hooks documentation.
Choose CommonJS or ESM deliberately
The first examples use CommonJS (require). Mocha also supports native ECMAScript modules. For ESM, either save the test with a .mjs extension or use a .js file in a package whose package.json sets "type": "module". An ESM test can import Node’s assertion module like this:
import assert from 'node:assert';
describe('Array#indexOf()', function () {
it('returns -1 when the value is absent', function () {
assert.strictEqual([1, 2, 3].indexOf(4), -1);
});
});Mocha’s documented limitation is that watch mode does not support ESM test files. Do not assume every plugin, reporter, or test mode behaves identically across module systems; check the relevant current documentation when adding one. See Mocha’s ESM guide.
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Make test settings repeatable
Start with npx mocha and add persistent configuration only when the project needs it. Mocha supports configuration in .mocharc.js, .mocharc.cjs, .mocharc.mjs, YAML, JSON or JSONC files, and in a mocha property in package.json. For example, a package-level property can select a test directory pattern:
{
"mocha": {
"spec": "test/**/*.js"
}
}When settings conflict, the precedence is command-line flags, then MOCHA_OPTIONS, then a configuration file, then the mocha property in package.json. A command-line flag is therefore suitable for a one-off override; a config file or package property is better for shared project defaults. See the configuration documentation.
Add CLI options to solve a specific need
The CLI reference describes a default spec reporter and a 2-second timeout; retries are opt-in. It also documents --parallel for running files in a worker pool and --watch for rerunning tests after changes. These are options, not requirements for every project. Parallel execution may be a poor fit for tests that share mutable external state, and watch mode has the ESM limitation noted above. Review the current Mocha CLI reference before relying on defaults or flags, since CLI behavior can change.
Troubleshoot common failures
- Mocha reports a Node.js version or syntax compatibility problem: compare
node --versionwith the documented v12 requirement above, and use a compatible runtime. - No tests are found: run the command from the project root and check that files are under
test/, or set the intended pattern in a supported config. - An asynchronous test finishes too early or hangs: make sure the test uses exactly one completion mechanism and that a callback calls
doneon both success and error paths. A Promise-based API should return or be awaited. - Mocha reports overspecified completion: remove either the returned Promise or the
donecallback; do not combine them in one test. - Tests pass alone but interfere with each other: move state reset into
beforeEachor cleanup intoafterEach, and avoid depending on test execution order. - ESM tests do not rerun in watch mode: Mocha documents that ESM test files are unsupported in watch mode; run them without watch or use an appropriate CommonJS arrangement if watch is necessary.
- A setting appears ignored: check for a higher-precedence command-line flag or
MOCHA_OPTIONSvalue before changing the config file.
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Frequently Asked Questions
Can Mocha run tests without an assertion library?
Yes. Node.js provides the built-in node:assert module used in this tutorial.
Quick Recap
Does Mocha run tests in parallel by default?
No. Parallel execution is an opt-in CLI mode.
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