To test a Node.js application with Mocha, install Mocha as a project development dependency, put test files in test/, write cases with describe and it, and run them with npx mocha. Choose one completion method for each asynchronous test—callback, Promise, or async/await—and use hooks when tests need setup or cleanup.
This guide uses Mocha’s documented v12 requirements and behavior; its compatibility requirement is Node.js ^20.19.0 || >=22.12.0, as of Mocha v12.0.0. See the official getting-started guide for the current installation instructions.
Check Node.js and install Mocha
Check your installed Node.js version before adding Mocha. The v12.0.0 getting-started documentation requires Node.js ^20.19.0 || >=22.12.0. This is a compatibility requirement, not a claim that any particular Mocha version is the latest.
node --versionInstall Mocha locally as a development dependency so the project records the test runner alongside its other development tools:
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npm i -D mochaWith pnpm or Yarn, the equivalent documented installation commands are:
pnpm add -D mocha
yarn add --dev mochaUse the package manager already used by your project; these commands install the same test runner and do not change the test-writing workflow.
Write and run your first test
Mocha’s default BDD interface provides describe to group related behavior and it to define a test case. Node.js includes an assertion library, so a separate assertion package is not required for a simple test.
Create test/array.test.js in a CommonJS project:
const assert = require('node:assert');
describe('Array#indexOf()', function () {
it('returns -1 when the value is not present', function () {
assert.strictEqual([1, 2, 3].indexOf(4), -1);
});
});Run the test from the project directory:
npx mochaMocha discovers tests in test/ by default. A passing result is reported by the runner; the official guide illustrates the output as 1 passing. That sample describes the guide’s example, not an independently executed result for your project.
Adapt the pattern to application code
The following function is illustrative: put a function in your application module, import it from a test, and assert its documented behavior. For a CommonJS project, for example:
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// src/discount.js
function discountedPrice(price, percentage) {
return price * (1 - percentage / 100);
}
module.exports = { discountedPrice };
// test/discount.test.js
const assert = require('node:assert');
const { discountedPrice } = require('../src/discount');
describe('discountedPrice', function () {
it('subtracts the specified percentage from the price', function () {
assert.strictEqual(discountedPrice(100, 20), 80);
});
});Keep the expected result tied to the behavior you intend to preserve. An assertion that only checks a function ran is less useful than one that checks its observable output.
Add a package test script
To make the command repeatable through the project’s normal package workflow, add a script to package.json:
{
"scripts": {
"test": "mocha"
}
}Then run npm test (or the corresponding test command for your package manager). This script is a convenience wrapper around Mocha; it does not replace Mocha’s CLI.
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Mocha waits for asynchronous work when a test calls its completion callback, returns a Promise, or is declared with async. Match the pattern to the API under test and use only one completion signal in a test. The same approaches are available to asynchronous hooks.
Callback API: use done
For a callback-style API, accept Mocha’s done callback and call it after the operation completes. Pass an error to done to fail the test:
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it('completes a callback operation', function (done) {
readValue((err, value) => {
if (err) return done(err);
try {
assert.strictEqual(value, 'ready');
done();
} catch (assertionError) {
done(assertionError);
}
});
});readValue is an illustrative callback API; replace it with the application function you are testing. Catching an assertion error inside the callback and forwarding it to done ensures a failed assertion is reported as a test failure.
Promise API: return the Promise
If the operation returns a Promise, return it from the test. Mocha treats fulfillment as completion and a rejection as failure:
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return fetchValue().then(function (value) {
assert.strictEqual(value, 'ready');
});
});fetchValue is illustrative. The important part is returning the Promise chain so Mocha can observe its result.
Promise API: use async/await
An async test returns a Promise automatically, so you can await the operation and assert its result directly:
it('resolves with the value', async function () {
const value = await fetchValue();
assert.strictEqual(value, 'ready');
});Do not combine a returned Promise with done() in one test. Mocha treats that as overspecified completion because the test signals completion twice.
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Use hooks for setup and cleanup
Hooks run code around tests in a suite. before and after run once for their suite; beforeEach and afterEach run around every test in that suite. Hooks may be synchronous or asynchronous.
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let cart;
beforeEach(function () {
cart = createCart();
});
afterEach(function () {
cart = null;
});
it('starts empty', function () {
assert.strictEqual(cart.items.length, 0);
});
it('accepts an item', function () {
cart.add({ sku: 'book', quantity: 1 });
assert.strictEqual(cart.items.length, 1);
});
});createCart is illustrative application setup. Creating fresh state in beforeEach keeps one test’s changes from leaking into the next. Use before and after for work genuinely shared across the suite, such as opening and closing a resource once; use per-test hooks when each test needs an isolated fixture.
For asynchronous setup or cleanup, return a Promise or make the hook async. Avoid leaving external resources open after tests: close them in cleanup even when later tests depend on the same suite.
describe('service', function () {
let service;
before(async function () {
service = await connectToService();
});
after(async function () {
await service.close();
});
});The service functions above are examples, not a database or service implementation. Keep hooks close to the suite they serve. For root-level hooks, Mocha’s documentation recommends Root Hook Plugins as the preferred mechanism since v8; see the hooks guide for details.
Choose CommonJS or ESM deliberately
The first examples use CommonJS (require). Mocha also supports native ECMAScript module tests. Use an .mjs test file, or use .js files in a package whose package.json sets "type": "module".
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ESM test example
For an ESM project, save the following as test/array.test.mjs:
import assert from 'node:assert';
import { describe, it } from 'mocha';
describe('Array#indexOf()', function () {
it('returns -1 when the value is not present', function () {
assert.strictEqual([1, 2, 3].indexOf(4), -1);
});
});Alternatively, save it as test/array.test.js under a package configured with "type": "module". The documented limitation is that Mocha watch mode does not support ESM test files. If you rely on plugins, custom reporters, or less common modes, check the current Mocha documentation for their module-format compatibility rather than assuming every setup behaves identically.
Keep configuration simple, then make it repeatable
Start with npx mocha. Add persistent configuration only when the project needs options shared across runs. Mocha supports configuration in several formats, including .mocharc.js, .mocharc.cjs, .mocharc.mjs, YAML, JSON, and JSONC, as well as a mocha property in package.json. The configuration documentation describes the available forms.
When settings overlap, Mocha applies this precedence from highest to lowest:
- Command-line flags
MOCHA_OPTIONSenvironment variable- Mocha configuration file
mochaproperty inpackage.json
That order lets a one-off command override shared defaults, while environment settings can adjust an invocation without editing the project configuration.
CLI options worth adding selectively
Mocha’s CLI reference documents a default spec reporter and a two-second timeout; retries are opt-in. It also documents parallel test-file execution with --parallel and rerunning tests after changes with --watch. Defaults and options are version-sensitive, so consult the CLI reference for the version installed in your project.
- Use
--timeoutwhen legitimate operations need more time, rather than treating a timeout as proof that the test passed. - Consider
--parallelonly when test files can run independently and the project is ready for worker-based execution. - Use
--watchfor a quick edit-and-rerun loop, keeping in mind the ESM watch-mode limitation. - Retries are not a substitute for fixing nondeterministic tests; enable them only when the retry behavior is appropriate to your suite.
Troubleshoot common failures
- Mocha will not start under your Node.js version: Compare
node --versionwith the v12.0.0 requirement^20.19.0 || >=22.12.0. Upgrade Node.js to a compatible release or use a Mocha version supported by your project’s runtime. - No tests are found: Check that files are under the default
test/directory, use a recognized test-file extension, and runnpx mochafrom the project root. If your tests live elsewhere, consult the CLI reference for how to specify them. - A callback test hangs: Ensure every successful callback path calls
done()and every error path passes the error todone(err). Also check that the underlying callback is actually invoked. - Mocha reports overspecified completion: Remove either the returned Promise or the
donecallback. A test must use one completion mechanism, not both. - An asynchronous assertion appears to pass unexpectedly: Return the Promise or make the test
asyncand await it. Otherwise Mocha may finish before the asynchronous assertion runs. - Setup works for one test but leaks state into another: Move per-test fixture creation and cleanup into
beforeEachandafterEach, rather than sharing mutable state with suite-level hooks. - ESM tests do not rerun in watch mode: Mocha documents that ESM test files are not supported by watch mode. Run the tests without watch mode or use a compatible test setup.
- A setting seems ignored: Check whether a higher-precedence CLI flag or
MOCHA_OPTIONSvalue overrides the config file orpackage.jsonsetting.
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