Code coverage with Mocha

Question

I am using Mocha for testing my NodeJS application. I am not able to figure out how to use its code coverage feature. I tried googling it but did not find any proper tutorial. Please help.

That

Answer 1

You need an additional library for code coverage, and you are going to be blown away by how powerful and easy istanbul is. Try the following, after you get your mocha tests to pass:

npm install nyc

Now, simply place the command nyc in front of your existing test command, for example:

{
  "scripts": {
    "test": "nyc mocha"
  }
}

Lytic

And if you're running a locally installed version of mocha, try istanbul cover node_modules/mocha/bin/_mocha. – Lipophilic

Or, install both istanbul and mocha locally, and add the following to the scripts section of your package.json and then just npm run coverage: "coverage": "./node_modules/istanbul/lib/cli.js cover ./node_modules/mocha/bin/_mocha -- --ui bdd -R spec -t 5000 – Lytic

has anyone tried this using grunt and coffeescript? I've looked into grunt-mochacov which works fine in linux but not windows (and its using blanket which didnt seem overly functional to me) – Lalittah

github.com/daniellmb/grunt-istanbul-coverage or github.com/taichi/grunt-istanbul should work for you. And here is Coffeescript and Istanbul: github.com/Constellation/ibrik – Lytic

I had trouble getting this command to run on windows, but by specifying the full path to the mocha bin I was able to get it to work. istanbul.cmd cover C:\Users\{UserName}\AppData\Roaming\npm\node_modules\mocha\bin\_mocha – Clearwing

@JasonJarrett I highly recommend installing Git Bash (most easily gotten via Github for WIndows). You'll find that most commands run correctly. – Lytic

$(npm bin) is a canonical shortcut to ./node_modules/.bin/, and istanbul/lib/cli.js is aliased to istanbul in the bin folder. So here's a shorter command: $(npm bin)/istanbul cover $(npm bin)/_mocha -- --ui bdd -R spec -t 5000 – Tailrace

@ Windows users: istanbul cover node_modules/mocha/bin/_mocha -- -R spec – Felspar

One more minor note to make this idiot proof might be the use of "--recursive" – Mckim

Please note that istanbul actually parses the AST, and therefor if you have comments in your code for various things these will be removed. I spent about 2 hours troubleshooting istanbul only to have to ultimately back out and find a new solution. – Intercurrent

You can also use npmjs.com/package/gulp-istanbul. It has option to specify npmjs.com/package/isparta as an instrumenter which provides code coverage for ES6. – Mind

Please note that "coverage":"istanbul cover _mocha" in package.json will not work, as it will refer to ./node_modules/.bin/_mocha instead of ./node_modules/mocha/bin/_mocha where first is a shell script and the later is a js – Coccidioidomycosis

It gives me this error: $ istanbul cover _mocha -- -R spec No coverage information was collected, exit without writing coverage information C:\Users\Ulysses\AppData\Roaming\npm\_mocha.CMD:1 (function (exports, require, module, __filename, __dirname) { @IF EXIST "%~dp0 ^ SyntaxError: Unexpected token ILLEGAL How can I solve this please? – Kinakinabalu

@UlyssesAlves Looks like you are running a windows OS, like @Pier-Luc Gendreau mentioned you will need specify istanbul cover node_modules/mocha/bin/_mocha -- -R spec Source: github.com/gotwarlost/istanbul/issues/90 istanbul assumes that the _mocha you specify translates to a JS file which is true in non-windows environments. In a windows environment, this is likely a batch file that is getting loaded by node and, of course, fails. – Sardinia

Answer 2

Now (2024) the preferred way to use istanbul is via its "state of the art command line interface" nyc.

Setup

First, install it in your project with

npm i nyc --save-dev

Then, if you have a npm based project, just change the test script inside the scripts object of your package.json file to execute code coverage of your mocha tests:

{
  "scripts": {
    "test": "nyc --reporter=text mocha"
  }
}

Run

Now run your tests

npm test

and you will see a table like this in your console, just after your tests output:

Customization

Html report

Just use

nyc --reporter=html

instead of text. Now it will produce a report inside ./coverage/index.html.

Report formats

Istanbul supports a wide range of report formats. Just look at its reports library to find the most useful for you. Just add a --reporter=REPORTER_NAME option for each format you want. For example, with

nyc --reporter=html --reporter=text

you will have both the console and the html report.

Don't run coverage with npm test

Just add another script in your package.json and leave the test script with only your test runner (e.g. mocha):

{
  "scripts": {
    "test": "mocha",
    "test-with-coverage": "nyc --reporter=text mocha"
  }
}

Now run this custom script

npm run test-with-coverage

to run tests with code coverage.

Force test failing if code coverage is low

Fail if the total code coverage is below 90%:

nyc --check-coverage --lines 90 

Fail if the code coverage of at least one file is below 90%:

nyc --check-coverage --lines 90 --per-file

Spinelli

This works perfectly for jasmine also : "nyc --reporter=html jasmine" – Bathroom

mine works but I do not see the code coverage showing which lines are covered by green / red in the code itself. – Methyl

thanks for adding "now(2017)" - really helpful in this fast moving javascript world – Neutral

In case anyone else was confused - the npm repository istanbul seems to have been superceded by nyc. Per its listed dependencies, istanbul was split apart into various packages which are all maintained in their istanbuljs monorepo – Aarika

I am having the --reporter=html enabled but the html file is empty always, there is nothing shown about uncovered blocks or % covered etc just headers of the table – Perilune

npm run test-with-coverage fails with (function (exports, require, module, __filename, __dirname) { import _ from 'lodash'^^^^^^. – Concatenation

Nice this is a useful response, @lifeisfoo; It seems to be more up-to-date than this medium article: medium.com/the-node-js-collection/… Does anyone have any thoughts on c8 vs istanbul? Just based on ease-of-use I'm leaning towards istanbul. – Mannuela

Make sure to put --reporter or other params of nyc right after nyc and not after mocha or whatever else you're using. – Gay

If your test get stuck after tests completion and don't show the coverage report. Add --exit parameter at the end. test-cover": "nyc mocha --exit" – Lyman

Note: This doens't work with ESM modules. C8 seem to be the new best option. – Endowment

Answer 3

The accepted answer (nyc) does not work if you are using ESM modules.

C8 appears to be the best solution now, which leverages built-in NodeJS capabilities and utilizes istanbul (like nyc, and shares the same config files).

npm install -g c8
c8 mocha

It will use .nycrc for configuration. A sample configuration I'm using is:

{
    "all": true,
    "exclude": ["test"],
    "output": "reports",
    "reporter" : [
        "html",
        "text"
    ]
}

(Note: I was pointed to c8 by an answer to another question https://mcmap.net/q/88224/-no-coverage-nyc-mocha)

Endowment

It worked really well for me, even with lots of added mocha parameters, e.g. from my package.json: { "scripts": { "test": "mocha --reporter=spec --recursive test/spec", "coverage": "c8 mocha --reporter=html --recursive test/spec", ... ... ... – Sanctum

Answer 4

Blanket.js works perfect too.

npm install --save-dev blanket

in front of your test/tests.js

require('blanket')({
    pattern: function (filename) {
        return !/node_modules/.test(filename);
    }
});

run mocha -R html-cov > coverage.html

Maunder

require('blanket')({ pattern: function (filename) { return !/node_modules/.test(filename); } }); – Maunder

As of 2015, blanket.js is not maintained anymore and doesn't support ES6. Istanbul is highly recommended. – Wentletrap