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How to write a test for the Node.js project

What is a test?

Most tests in Node.js core are JavaScript programs that exercise a functionality provided by Node.js and check that it behaves as expected. Tests should exit with code 0 on success. A test will fail if:

  • It exits by setting process.exitCode to a non-zero number.
    • This is usually done by having an assertion throw an uncaught Error.
    • Occasionally, using process.exit(code) may be appropriate.
  • It never exits. In this case, the test runner will terminate the test because it sets a maximum time limit.

Add tests when:

  • Adding new functionality.
  • Fixing regressions and bugs.
  • Expanding test coverage.

Test directory structure

See directory structure overview for outline of existing test & locations. When deciding on whether to expand an existing test file or create a new one, consider going through the files related to the subsystem. For example, look for test-streams when writing a test for lib/streams.js.

Test structure

Let's analyze this basic test from the Node.js test suite:

'use strict';                                                          // 1
const common = require('../common');                                   // 2
const fixtures = require('../common/fixtures');                        // 3

// This test ensures that the http-parser can handle UTF-8 characters  // 5
// in the http header.                                                 // 6

const assert = require('assert');                                      // 8
const http = require('http');                                          // 9

const server = http.createServer(common.mustCall((req, res) => {       // 11
  res.end('ok');                                                       // 12
}));                                                                   // 13
server.listen(0, () => {                                               // 14
  http.get({                                                           // 15
    port: server.address().port,                                       // 16
    headers: { 'Test': 'Düsseldorf' }                                  // 17
  }, common.mustCall((res) => {                                        // 18
    assert.strictEqual(res.statusCode, 200);                           // 19
    server.close();                                                    // 20
  }));                                                                 // 21
});                                                                    // 22
// ...                                                                 // 23

Lines 1-3

'use strict';
const common = require('../common');
const fixtures = require('../common/fixtures');

The first line enables strict mode. All tests should be in strict mode unless the nature of the test requires that the test run without it.

The second line loads the common module. The common module is a helper module that provides useful tools for the tests. Some common functionality has been extracted into submodules, which are required separately like the fixtures module here.

Even if a test uses no functions or other properties exported by common, the test should still include the common module before any other modules. This is because the common module includes code that will cause a test to fail if the test leaks variables into the global space. In situations where a test uses no functions or other properties exported by common, include it without assigning it to an identifier:


Lines 5-6

// This test ensures that the http-parser can handle UTF-8 characters
// in the http header.

A test should start with a comment containing a brief description of what it is designed to test.

Lines 8-9

const assert = require('assert');
const http = require('http');

The test checks functionality in the http module.

Most tests use the assert module to confirm expectations of the test.

The require statements are sorted in ASCII order (digits, upper case, _, lower case).

Lines 11-22

This is the body of the test. This test is simple, it just tests that an HTTP server accepts non-ASCII characters in the headers of an incoming request. Interesting things to notice:

  • If the test doesn't depend on a specific port number, then always use 0 instead of an arbitrary value, as it allows tests to run in parallel safely, as the operating system will assign a random port. If the test requires a specific port, for example if the test checks that assigning a specific port works as expected, then it is ok to assign a specific port number.
  • The use of common.mustCall to check that some callbacks/listeners are called.
  • The HTTP server closes once all the checks have run. This way, the test can exit gracefully. Remember that for a test to succeed, it must exit with a status code of 0.

General recommendations


Avoid timers unless the test is specifically testing timers. There are multiple reasons for this. Mainly, they are a source of flakiness. For a thorough explanation go here.

In the event a test needs a timer, consider using the common.platformTimeout() method. It allows setting specific timeouts depending on the platform:

const timer = setTimeout(fail, common.platformTimeout(4000));

will create a 4-second timeout on most platforms but a longer timeout on slower platforms.

The common API

Make use of the helpers from the common module as much as possible. Please refer to the common file documentation for the full details of the helpers.


One interesting case is common.mustCall. The use of common.mustCall may avoid the use of extra variables and the corresponding assertions. Let's explain this with a real test from the test suite.

'use strict';
const assert = require('assert');
const http = require('http');

let request = 0;
let response = 0;
process.on('exit', function() {
  assert.equal(request, 1, 'http server "request" callback was not called');
  assert.equal(response, 1, 'http request "response" callback was not called');

const server = http.createServer(function(req, res) {
}).listen(0, function() {
  const options = {
    agent: null,
    port: this.address().port
  http.get(options, function(res) {

This test could be greatly simplified by using common.mustCall like this:

'use strict';
const common = require('../common');
const http = require('http');

const server = http.createServer(common.mustCall(function(req, res) {
})).listen(0, function() {
  const options = {
    agent: null,
    port: this.address().port
  http.get(options, common.mustCall(function(res) {

Countdown Module

The common Countdown module provides a simple countdown mechanism for tests that require a particular action to be taken after a given number of completed tasks (for instance, shutting down an HTTP server after a specific number of requests).

const Countdown = require('../common/countdown');

const countdown = new Countdown(2, function() {

countdown.dec(); // The countdown callback will be invoked now.

Testing promises

When writing tests involving promises, it is generally good to wrap the onFulfilled handler, otherwise the test could successfully finish if the promise never resolves (pending promises do not keep the event loop alive). The common module automatically adds a handler that makes the process crash - and hence, the test fail - in the case of an unhandledRejection event. It is possible to disable it with common.disableCrashOnUnhandledRejection() if needed.

const common = require('../common');
const assert = require('assert');
const fs = require('fs').promises;

// Wrap the `onFulfilled` handler in `common.mustCall()`.
    (content) => assert.strictEqual(content.toString(), 'test2')


Some tests will require running Node.js with specific command line flags set. To accomplish this, add a // Flags: comment in the preamble of the test followed by the flags. For example, to allow a test to require some of the internal/* modules, add the --expose-internals flag. A test that would require internal/freelist could start like this:

'use strict';

// Flags: --expose-internals

const assert = require('assert');
const freelist = require('internal/freelist');


When writing assertions, prefer the strict versions:

  • assert.strictEqual() over assert.equal()
  • assert.deepStrictEqual() over assert.deepEqual()

When using assert.throws(), if possible, provide the full error message:

  () => {
    throw new Error('Wrong value');
  /^Error: Wrong value$/ // Instead of something like /Wrong value/

ES.Next features

For performance considerations, we only use a selected subset of ES.Next features in JavaScript code in the lib directory. However, when writing tests, for the ease of backporting, it is encouraged to use those ES.Next features that can be used directly without a flag in all maintained branches. lists available features in each release, such as:

  • let and const over var
  • Template literals over string concatenation
  • Arrow functions when appropriate

Naming Test Files

Test files are named using kebab casing. The first component of the name is test. The second is the module or subsystem being tested. The third is usually the method or event name being tested. Subsequent components of the name add more information about what is being tested.

For example, a test for the beforeExit event on the process object might be named test-process-before-exit.js. If the test specifically checked that arrow functions worked correctly with the beforeExit event, then it might be named test-process-before-exit-arrow-functions.js.

Imported Tests

Web Platform Tests

Some of the tests for the WHATWG URL implementation (named test-whatwg-url-*.js) are imported from the Web Platform Tests Project. These imported tests will be wrapped like this:

/* The following tests are copied from WPT. Modifications to them should be
   upstreamed first. Refs:
/* eslint-disable */

// Test code

/* eslint-enable */

To improve tests that have been imported this way, please send a PR to the upstream project first. When the proposed change is merged in the upstream project, send another PR here to update Node.js accordingly. Be sure to update the hash in the URL following WPT Refs:.

C++ Unit test

C++ code can be tested using Google Test. Most features in Node.js can be tested using the methods described previously in this document. But there are cases where these might not be enough, for example writing code for Node.js that will only be called when Node.js is embedded.

Adding a new test

The unit test should be placed in test/cctest and be named with the prefix test followed by the name of unit being tested. For example, the code below would be placed in test/cctest/

#include "gtest/gtest.h"
#include "node_test_fixture.h"
#include "env.h"
#include "node.h"
#include "v8.h"

static bool called_cb = false;
static void at_exit_callback(void* arg);

class EnvTest : public NodeTestFixture { };

TEST_F(EnvTest, RunAtExit) {
  v8::HandleScope handle_scope(isolate_);
  v8::Local<v8::Context> context = v8::Context::New(isolate_);
  node::IsolateData* isolateData = node::CreateIsolateData(isolate_, uv_default_loop());
  Argv argv{"node", "-e", ";"};
  auto env = node::CreateEnvironment(isolateData, context, 1, *argv, 2, *argv);
  node::AtExit(env, at_exit_callback);

static void at_exit_callback(void* arg) {
  called_cb = true;

Next add the test to the sources in the cctest target in node.gyp:

'sources': [

Note that the only sources that should be included in the cctest target are actual test or helper source files. There might be a need to include specific object files that are compiled by the node target and this can be done by adding them to the libraries section in the cctest target.

The test can be executed by running the cctest target:

$ make cctest

A filter can be applied to run single/multiple test cases:

$ make cctest GTEST_FILTER=EnvironmentTest.AtExitWithArgument

cctest can also be run directly which can be useful when debugging:

$ out/Release/cctest --gtest_filter=EnvironmentTest.AtExit*

Node.js test fixture

There is a test fixture named node_test_fixture.h which can be included by unit tests. The fixture takes care of setting up the Node.js environment and tearing it down after the tests have finished.

It also contains a helper to create arguments to be passed into Node.js. It will depend on what is being tested if this is required or not.

Test Coverage

To generate a test coverage report, see the Test Coverage section of the Pull Requests guide.