writing-tests.md

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.

Tests can be added for multiple reasons:

• When adding new functionality.
• When fixing regressions and bugs.
• When expanding test coverage.

Test structure

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

1   'use strict';
2   const common = require('../common');
3
4   // This test ensures that the http-parser can handle UTF-8 characters
5   // in the http header.
6
7   const http = require('http');
8   const assert = require('assert');
9
10  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 });

Lines 1-2

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

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.

Even if no functions or other properties exported by common are used in a test, the common module should still be included. This is because the common module includes code that will cause tests to fail if variables are leaked into the global space. In situations where no functions or other properties exported by common are used, it can be included without assigning it to an identifier:

require('../common');

Lines 4-5

// 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 7-8

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

These modules are required for the test to run. Except for special cases, these modules should only include core modules. The assert module is used by most of the tests to check that the assumptions for the test are met.

Lines 10-21

This is the body of the test. This test is quite 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 be 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 is closed 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

Timers

The use of timers is discouraged, unless timers are being tested. There are multiple reasons for this. Mainly, they are a source of flakiness. For a thorough explanation go here.

In the event a timer is needed, it's recommended using the common.platformTimeout() method, that allows setting specific timeouts depending on the platform. For example:

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

will create a 4-seconds timeout, except for some platforms where the delay will be multiplied for some factor.

The common API

Make use of the helpers from the common module as much as possible.

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';
var common = require('../common');
var assert = require('assert');
var http = require('http');

var request = 0;
var 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');
});

var server = http.createServer(function(req, res) {
  request++;
  res.end();
}).listen(0, function() {
  var options = {
    agent: null,
    port: this.address().port
  };
  http.get(options, function(res) {
    response++;
    res.resume();
    server.close();
  });
});

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

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

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

Flags

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

'use strict';

// Flags: --expose-internals

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

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.