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mocha-parallel-tests is a test runner for tests written with mocha testing framework which allows you to run them in parallel. mocha-parallel-tests executes each of your test files in a separate process while maintaining the output structure of mocha. Compared to the other tools which try to parallelize mocha tests execution, mocha-parallel-tests doesn't require you to write the code in a different way or use some specific APIs - just run your tests with mocha-parallel-tests instead of mocha and you will see the difference. Or if you prefer to use mocha programmatic API replace it with mocha-parallel-tests default export and you're done!

If you're using Node.JS >= 12 your tests execution will be even faster because mocha-parallel-tests supports running tests with Node.JS worker threads API: instead of creating fully fledged Node.JS processes mocha-parallel-tests runs your tests in lighter threads within the same process. This results in a faster tests processing and less memory consumption.


npm install --save-dev mocha mocha-parallel-tests

ATTENTION: mocha is a peer dependency of mocha-parallel-tests so you also need to install mocha. Currently mocha versions 3, 4, 5, 6, and 7 are supported.



# mocha example
$ mocha -R xunit --timeout 10000 --slow 1000 test/*.spec.js

# mocha-parallel-tests example
$ mocha-parallel-tests -R xunit --timeout 10000 --slow 1000 test/*.spec.js

Most of mocha CLI options are supported. If you're missing some of the options support you're welcome to submit a PR: all options are applied in a same simple way.

Programmatic API

// mocha example
import Mocha from 'mocha';
const mocha = new Mocha();

// mocha-parallel-tests example
// if you're using TypeScript you don't need to install @types/mocha-parallel-tests
// because package comes with typings in it
import Mocha from 'mocha-parallel-tests'; // or `const Mocha = require('mocha-parallel-tests').default` if you're using CommonJS
const mocha = new Mocha();

Parallel limit

mocha-parallel-tests CLI executable has its own --max-parallel option which is the amount of tests executed at the same time. By default it's equal to the number of logical CPI cores (os.cpus().length) on your computer but you can also specify your own number or set it to 0, which means that all test files will be started executing at the same time. However this is not recommended especially on machines with low number of CPUs and big number of tests executed.

Differences with mocha

Main difference with mocha comes from the fact that all files are executed in separate processes/threads and don't share the scope. This means that even global variables values that you could've used to share the data between test suites will not be reliable. There's also some specific behaviour for some of the mocha CLI options like --bail: it's just applied to each test in its process. You can see the full list of differences here.

There's also a list of limitations that you can hit when you launch your tests with mocha-parallel-tests.

From the reporter perspective the main difference between tests executed with mocha and mocha-parallel-tests is another level of nesting which again comes from the fact that main process adds one more "suite" level and all tests results are merged into that:


mocha spec reporter output


mocha-parallel-tests spec reporter output

Comparison with mocha.parallel

mocha.parallel is a tool which allows you to run mocha tests in parallel. While it seems pretty similar to mocha-parallel-tests there's a massive difference between them. Check this page for a full comparison.