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18.0.2 • Public • Published

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Node test utility

Build Status

Lead Maintainer: Wyatt Preul

lab is sponsored by Joyent.


lab is a simple test utility for Node.js. Unlike other test utilities, lab uses only async/await features and includes everything you should expect from a modern Node.js test utility. Our goal with lab is to keep the execution engine as simple as possible, and not try to build an extensible framework.

lab works with any assertion library that throws an error when a condition isn't met.

Command Line

lab supports the following command line options:

  • -a, --assert - name of assert library to use. To disable assertion library set to false.
  • --bail - terminate the process with a non-zero exit code on the first test failure. Defaults to false.
  • -c, --coverage - enables code coverage analysis.
  • --coverage-path - sets code coverage path.
  • --coverage-exclude - sets code coverage excludes.
  • --coverage-all - report coverage for all matched files, not just those tested.
  • --coverage-flat - do not perform a recursive find of files for coverage report. Requires --coverage-all
  • --coverage-pattern - only report coverage for files with the given pattern in the name. Defaults to pattern. Requires --coverage-all
  • -C, --colors - enables or disables color output. Defaults to console capabilities.
  • -d, --dry - dry run. Skips all tests. Use with -v to generate a test catalog. Defaults to false.
  • -e, --environment - value to set the NODE_ENV environment variable to, defaults to 'test'.
  • -f, --flat - do not perform a recursive load of test files within the test directory.
  • -g, --grep - only run tests matching the given pattern which is internally compiled to a RegExp.
  • -h, --help - show command line usage.
  • -i, --id - only run the test for the given identifier (or identifiers range, e.g. lab -i 1-3,5). Use lab -dv to print all tests and their identifier without running the tests.
  • -I, --ignore - ignore a list of globals for the leak detection (comma separated), this is an alias of globals property in .labrc file
  • --inspect - start lab in debug mode using the V8 Inspector.
  • -l, --leaks - disables global variable leak detection.
  • -L, --lint - run linting rules using linter. Disabled by default.
  • --lint-errors-threshold - maximum absolute amount of linting errors. Defaults to 0.
  • --lint-warnings-threshold - maximum absolute amount of linting warnings. Defaults to 0.
  • --lint-fix - apply any fixes from the linter, requires -L or --lint to be enabled. Disabled by default.
  • --lint-options - specify options to pass to linting program. It must be a string that is JSON.parse(able).
  • -m, --timeout - individual tests timeout in milliseconds (zero disables timeout). Defaults to 2 seconds.
  • -M, --context-timeout - default timeouts for before, after, beforeEach and afterEach in milliseconds. Disabled by default.
  • -o, --output - file to write the report to, otherwise sent to stdout.
  • -p, --default-plan-threshold - sets the minimum number of assertions a test must run. Overridable with plan.
  • -P, --pattern - only load files with the given pattern in the name.
  • -r, --reporter - the reporter used to generate the test results. Defaults to console. Options are:
    • console - text report.
    • html - HTML test and code coverage report (sets -c).
    • json - output results in JSON format.
    • junit - output results in JUnit XML format.
    • tap - TAP protocol report.
    • lcov - output to lcov format.
    • clover - output results in Clover XML format.
    • Multiple Reporters - See Below
    • Custom Reporters - See Below
  • --shuffle - randomize the order that test scripts are executed. Will not work with --id.
  • --seed - use this seed to randomize the order with --shuffle. This is useful to debug order dependent test failures.
  • -s, --silence - silence test output, defaults to false.
  • -S, --sourcemaps - enables sourcemap support for stack traces and code coverage, disabled by default.
  • -t, --threshold - sets the minimum code test coverage percentage to 100%.
  • -T, --transform - javascript file that exports an array of objects ie. [ { ext: ".js", transform: (content, filename) => { ... } } ]. Note that if you use this option with -c (--coverage), then you must generate sourcemaps and pass sourcemaps option to get proper line numbers.
  • -v, --verbose - verbose test output, defaults to false.
  • -V, --version - display lab version information.


To install locally:

$ npm install --save-dev lab

By default, lab loads all the '*.js' files inside the local 'test' directory and executes the tests found. To use different directories or files, pass the file or directories as arguments:

$ lab unit.js

Test files must require the lab module, and export a test script:

const { expect } = require('code');
const { it } = exports.lab = require('lab').script();
it('returns true when 1 + 1 equals 2', () => {
    expect(1 + 1).to.equal(2);


const { expect } = require('code');
const Lab = require('lab');
const lab = exports.lab = Lab.script();
lab.test('returns true when 1 + 1 equals 2', () => {
    expect(1 + 1).to.equal(2);

If a test is performing an asynchronous operation then it should use the new async/await keywords or return a Promise. For example:

lab.test('config file has correct value', async () => {
    const file = await fs.readFile('config');

Tests can be organized into experiments:

lab.experiment('math', () => {
    lab.test('returns true when 1 + 1 equals 2', () => {
        expect(1 + 1).to.equal(2);

If you need to perform some async actions before or after executing the tests inside an experiment, the before() and after() methods can be used. To execute code before or after each test in an experiment, use beforeEach() and afterEach().

lab.experiment('math', () => {
    lab.before(() => {
        return new Promise((resolve) => {
            // Wait 1 second
            setTimeout(() => {
            }, 1000);
    lab.beforeEach(() => {
        // Run before every single test
    lab.test('returns true when 1 + 1 equals 2', () => {
        expect(1 + 1).to.equal(2);

test(), before(), beforeEach(), after() and afterEach() also support returning promises just as tests do:

lab.experiment('math', () => {
    lab.before(() => {
        return aFunctionReturningAPromise();
    lab.test('returns true when 1 + 1 equals 2', () => {
        return aFunctionReturningAPromise()
            .then((aValue) => {

Both test() and experiment() accept an optional options argument which must be an object with the following optional keys:

  • timeout - set a test or experiment specific timeout in milliseconds. Defaults to the global timeout (2000ms or the value of -m).
  • skip - skip execution. Cannot be overridden in children once parent is set to skip.
  • only - marks all other tests or experiments with skip.

You can also append .only(…) or .skip(…) to test and experiment instead of using options:

lab.experiment('with only', () => {
    lab.test.only('only this test will run', () => {
        expect(1 + 1).to.equal(2);
    lab.test('another test that will not be executed', () => {});


The test function options support a plan property, used to specify the expected number of assertions for your test to execute. This setting should only be used with an assertion library that supports a count() function, like code.

lab.experiment('my plan', () => {
    lab.test('only a single assertion executes', { plan: 1 }, () => {
        expect(1 + 1).to.equal(2);


The test function is passed a flags object that can be used to create notes or set a function to execute for cleanup operations after the test is complete.


Notes are included in the console reporter at the end of the output. For example, if you would like to add a note with the current time, your test case may look like the following:

lab.test('attaches notes', (flags) => {
    expect(1 + 1).to.equal(2);
    flags.note(`The current time is ${}`);

Multiple notes can be appended for the same test case by simply calling note() repeatedly.


Declare that a particular function must be called a certain number of times. The signature to mustCall is (fn, numberOfExecutions) and it returns a wrapped copy of the fn. After the test is complete, each mustCall assertion will be checked and the test will fail if any function was called the incorrect number of times.

Below is an example demonstrating how to use mustCall to verify that fn is called exactly two times.

lab.test('fn must be called twice', async (flags) => {
    const fn = () => {};
    const wrapped = flags.mustCall(fn, 2);
    await doSomeAsyncOperation();


You can assign a function to the flags object onCleanup property to register a runtime cleanup function to be executed after the test completed. The cleanup function will execute even in the event of a timeout. Note that the cleanup function will be executed as-is without any timers. Like the test, onCleanup can return a Promise that will be evaluated.

lab.test('cleanups after test', (flags) => {
    flags.onCleanup = () => {
    expect(1 + 1).to.equal(2);


You can assign a synchronous function to the flags object onUnhandledRejection property to register an override for global rejection handling. This can be used to test the code that is explicitly meant to result in unhandled rejections.

lab.test('leaves an unhandled rejection', (flags) => {
    return new Promise((resolve) => {
        flags.onUnhandledRejection = (err) => {
            expect(err)'I want this rejection to remain unhandled in production');
            resolve(); // finish the test
        // sample production code
        setTimeout(() => {
            Promise.reject(new Error('I want this rejection to remain unhandled in production'));


You can assign a synchronous function to the flags object onUncaughtException property to register an override for global exception handling. This can be used to test the code that is explicitly meant to result in uncaught exceptions.

lab.test('leaves an uncaught rejection', (flags) => {
    return new Promise((resolve) => {
        flags.onUncaughtException = (err) => {
            expect(err)'I want this exception to remain uncaught in production');
            resolve(); // finish the test
        // sample production code
        setTimeout(() => {
            throw new Error('I want this exception to remain uncaught in production');


context is an object that is passed to before and after functions in addition to tests themselves. The intent is to be able to set properties on context inside of a before function that can be used by a test function later. This should help reduce module level variables that are set by before/beforeEach functions. Tests aren't able to manipulate the context object for other tests.

lab.before(({ context }) => { = 'bar';
lab.test('contains context', ({ context }) => {


before(), after(), beforeEach(), afterEach() accept an optional options argument which must be an object with the following optional keys:

  • timeout - set a specific timeout in milliseconds. Disabled by default or the value of -M.
lab.experiment('math', { timeout: 1000 }, () => {
    lab.before({ timeout: 500 }, () =>  {
    lab.test('returns true when 1 + 1 equals 2', () =>  {
        expect(1 + 1).to.equal(2);

Script options

The script([options]) method takes an optional options argument where options is an object with the following optional keys:

  • schedule - if false, an automatic execution of the script is disabled. Automatic execution allows running lab test scripts directly with Node.js without having to use the CLI (e.g. node test/script.js). When using lab programmatically, this behavior is undesired and can be turned off by setting schedule to false. If you need to see the output with schedule disabled you should set output to process.stdout. Defaults to true.
  • cli - allows setting command line options within the script. Note that the last script file loaded wins and usage of this is recommended only for temporarily changing the execution of tests. This option is useful for code working with an automatic test engine that run tests on commits. Setting this option has no effect when not using the CLI runner. For example setting cli to { ids: [1] } will only execute the first test loaded.

Behavior Driven Development

To make lab look like BDD:

const { expect } = require('code');
const Lab = require('lab');
const { after, before, describe, it } = exports.lab = Lab.script();
describe('math', () => {
    before(() => {});
    after(() => {});
    it('returns true when 1 + 1 equals 2', () => {
        expect(1 + 1).to.equal(2);

Test Driven Development

To make lab look like TDD:

const { expect } = require('code');
const Lab = require('lab');
const { suite, test } = exports.lab = Lab.script();
suite('math', () => {
    test('returns true when 1 + 1 equals 2', () => {
        expect(1 + 1).to.equal(2);


To use source transforms, you must specify a file with the -T command line option that tells Lab how to do the transformation. You can specify many extensions with different transform functions such as .ts or .jsx.


A TypeScript definition file is included with lab to make it easier to use inside of an existing TypeScript project. Below is a TypeScript test example that uses the lab-transform-typescript module to manage the transform:

import * as Lab from 'lab';
const { expect } = require('code');
const lab = Lab.script();
const { describe, it, before } = lab;
export { lab };
describe('experiment', () => {
    before(() => {});
    it('verifies 1 equals 1', () => {

Then the test can be be executed using the following command line:

$ lab --sourcemaps --transform node_modules/lab-transform-typescript

Disable Code Coverage

Sometimes you want to disable code coverage for specific lines, and have the coverage report omit them entirely. To do so, use the $lab:coverage:(off|on)$ comments. For example:

// There is no way to cover this in node 0.10
/* $lab:coverage:off$ */
if (typeof value === 'symbol') {
    // do something with value
/* $lab:coverage:on$ */

.labrc.js file

lab supports a .labrc.js configuration file for centralizing lab settings. The .labrc.js file can be located in the current working directory, any directory that is the parent of the current working directory, or in the user's home directory. The .labrc.js file needs to be able to be required by Node.js. Therefore, either format it as a JSON file or with a module.exports that exports an object with the keys that are the settings.

Below is an example of a .labrc.js file to enable linting and test coverage checking:

module.exports = {
    coverage: true,
    threshold: 90,
    lint: true

.labrc.js setting precedent

The .labrc.js file will override the lab default settings. Any options passed to the lab runner will override the settings found in .labrc.js. For example, assume you have the following .labrc.js file:

module.exports = {
    coverage: true,
    threshold: 100

If you need to reduce the coverage threshold for a single run, you can execute lab as follows:

lab -t 80

.labrc.js available settings

The .labrc.js file supports configuration keys that are named with the long name of the command line settings. Therefore, if you need to specify an assert library, you would export a key named "assert" with the desired value.

In addition, you can use the paths parameter to override the default test directory (i.e. ./test):

module.exports = {
    paths: ['test/lab'],

As stated at the beginning of the document, --ignore parameter is an alias for globals option in the .labrc file. Therefore if you wish to ignore specific files you'll need to append a globals setting, not an ignore one, as stated on #641.

Extending the linter

lab uses a shareable eslint config, and a plugin containing several hapi specific linting rules. If you want to extend the default linter you must:

  1. Add eslint-plugin-hapi and eslint-config-hapi as dependencies in your package.json. You must add both the plugin and the config because eslint treats them as peer dependencies. For more background, see eslint/eslint#3458 and eslint/eslint#2518.

  2. In your project's eslint configuration, add "extends": "eslint-config-hapi". eslint will automatically infer the eslint-config-, so technically you can just write "extends": "hapi".

Your project's eslint configuration will now extend the default lab configuration.

Ignoring files in linting

Since eslint is used to lint, you can create an .eslintignore containing paths to be ignored:


Only run linting

In order to run linting and not to execute tests you can combine the dry run flag with the lint flag.

lab -dL

Integration with an assertion library

Using the --assert argument allows you to integrate Lab with your favorite assertion library. Aside from --assert from the CLI you can change the assert option when executing report. Whatever assertion library you specify is imported and assigned to the Lab.assertions property. Here is an example using lab --assert code:

const lab = exports.lab = Lab.script();
const { describe, it } = lab;
// Testing shortcuts
const { expect, fail } = require('code');
describe('expectation', () => {
    it('should be able to expect', () => {
    it('should be able to fail (This test should fail)', () => {
        fail('Should fail');
$ lab --assert code

If you use the Code assertion library Lab will let you know if you have any missing assertions. An example of this is:

describe('expectation', () => {
    it('Test should pass but get marked as having a missing expectation', () => {
        // Invalid and missing assertion - false is a method, not a property!
        // This test will pass.

This is an invalid test but it will pass as the .false assertion was not actually called. Lab will report the number of incomplete assertions, their location in your code and return a failure of the tests.

Similarly, if you use an assertion library, lab will be able to report the verbosity of your tests. This is a measure of the number of assertions divided by the number of tests. The value will be output when using the console reporter and can be helpful in determining if too many or too few assertions exist in each test. What is too many or too few assertions is entirely up to you.


lab can be started with the option --inspect which will run it with the V8 Inspector.

This debugger can be accessed using the URL that is printed in the console, or used in association with a few Chrome extensions (Node.js V8 Inspector, NIM, etc).

As you may know, if your tests are associated with the command npm test, you can already run npm test -- --inspect to run it with the inspector and avoid creating another command. If you want to listen on a specific port for the inspector, pass --inspect={port}.

lab also has automatic support for the WebStorm debugger, just start a normal debugging session on your npm test script.

Best practices

  • Add lab as a dev dependency to your project's package.json along with a test script:
  "devDependencies": {
    "lab": "15.x.x"
  "scripts": {
    "test": "lab -t 100",
    "test-cov-html": "lab -r html -o coverage.html"

Note that npm test will execute lab with the -t 100 option which will require 100% code coverage. Run npm run test-cov-html and check the coverage.html file to figure out where coverage is lacking. When coverage is below the threshold, the CLI will exit with code 1 and will result in an npm Error message.

  • Run your tests with
$ npm test

Multiple Reporters

Multiple reporters can be specified by providing multiple reporter options.

$ lab -r console -r html

If any output -o is provided, they must match the same number of provided reporter options. The reporters would be paired with an output based on the order in which they were supplied. When specifying multiple outputs, use stdout to send a particular reporter to stdout.

$ lab -r console -o stdout -r html -o coverage.html -r lcov -o -r json -o data.json

In a .labrc.js file, multiple reporters and their associated output paths would be represented as follows:

module.exports = {
    reporter: ['console', 'html', 'lcov', 'json'],
    output: ['stdout', 'coverage.html', '', 'data.json']

Multiple reporters of the same kind are also supported.

$ lab -r console -o stdout -r console -o console.log

Custom Reporters

If the value passed for reporter isn't included with Lab, it is loaded from the filesystem. If the string starts with a period ('./custom-reporter'), it will be loaded relative to the current working directory. If it doesn't start with a period ('custom-reporter'), it will be loaded from the node_modules directory, just like any module installed using npm install.

Reporters must be a class with the following methods: start, test and end. options are passed to the class constructor upon initialization.

See the json reporter for a good starting point.

Excluding paths from coverage reporting

The --coverage-exclude argument can be repeated multiple times in order to add multiple paths to exclude. By default the node_modules and test directories are excluded. If you want to exclude those as well as a directory named public you can run lab as follows:

lab -c --coverage-exclude test --coverage-exclude node_modules --coverage-exclude public


lab initial code borrowed heavily from mocha, including the actual code used to render the coverage report into HTML. lab coverage code was originally adapted from blanket which in turn uses falafel.



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