Code coverage with Mocha - node.js

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.

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"
}
}

Answer 2

Now (2023) 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

Answer 3

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

Answer 4

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://stackoverflow.com/a/69846825/1949430)