[node.js] Code coverage with Mocha

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.

Now (2021) 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

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