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How to write a test for the Node.js project

What is a test?

A test must be a node script that exercises a specific functionality provided by node and checks that it behaves as expected. It should return 0 on success, otherwise it will fail. A test will fail if:

It exits by calling process.exit(code) where code != 0
It exits due to an uncaught exception.
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  const http = require('http');
4  const assert = require('assert');
5
6  const server = http.createServer(common.mustCall((req, res) => {
7    res.end('ok');
8  }));
9  server.listen(0, () => {
10   http.get({
11     port: server.address().port,
12     headers: {'Test': 'Düsseldorf'}
13   }, common.mustCall((res) => {
14     assert.equal(res.statusCode, 200);
15     server.close();
16   }));
17 });

Lines 1-2

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

These two lines are mandatory and should be included on every test. The common module is a helper module that provides useful tools for the tests. If for some reason, no functionality from common is used, it should still be included like this:

require('../common');

Why? It checks for leaks of globals.

Lines 3-4

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 6-17

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');

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