NodeJS Code Coverage With C8

Code coverage allows you to determine how well unit-tests cover your codebase. The standard code coverage suite for JavaScript is the NYC interface for Istanbul. However, this package doesn’t play well with NodeJS modules. Instead we are going to use the C8 package. C8 uses functionality native to NodeJS to create output that is compatible with Istanbul reporters.


npm i --save-dev c8


You can run c8 on any source file by simply passing in the appropriate node command.

$ npx c8 node src/index.js
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
All files  |   88.23 |      100 |      50 |   88.23 |
 MooYou.js |   84.61 |      100 |      50 |   84.61 | 9-10
 index.js  |     100 |      100 |     100 |     100 |

You can also easily run it with your Mocha test files.

$ npx c8 mocha
  MooYou Class Test
      ✔ returns 'you'

  1 passing (3ms)

File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
All files  |   84.61 |      100 |      50 |   84.61 |
 MooYou.js |   84.61 |      100 |      50 |   84.61 | 9-10


C8 can be configured from the command line, in your package.json file, or in a configuration file in your project directory. In particular you can instruct C8 to report one files as opposed to just those in your tests (npx c8 --all mocha). You can also specify which files to include and/or ignore. See the configuration documentation for specific details.

Generating Reports

By default the coverage data will be located in the coverage/tmp directory. You can change this directory with the --temp-directory flag. Don’t forget to add this to your .gitignore file.

Run npx c8 report to regenerate reports after c8 has already been run. Use the -r flag to specify which reporter to use, and the -o flag to specify where to output the report. By default generated reports can be found in the coverage/ dirctory.

Vanilla C8 command with HTML report:

npx c8 -r html mocha

Generate C8 report after the fact:

npx c8 -r html report

Two-part C8 command with custom directories and HTML report:

npx c8 --temp-directory .c8 mocha
npx c8 report --temp-directory .c8 -o html_coverage -r html

In Code Markup

You can add comments into your source code that tells c8 to ignore certain portions of code.

Ignoring all lines until told to stop.

/* c8 ignore start */
function foo() {
/* c8 ignore stop */

Ignore next line.

/* c8 ignore next */

External References

Git: Checkout a Single File from a Previous Commit

Often we alter or delete a file that we didn’t really want to. Sometimes this goes unnoticed for a few commits. It’s fairly easy in git to retrieve a specific file from a specific commit.

git checkout <commit> <path>
git checkout f08b32 ./src/main.c
git checkout HEAD~2 ./src/main.c

How to reattach a GIT detached head.

$ git status
HEAD detached at 36bc359
nothing to commit, working tree clean

$ git branch fix-detached
$ git checkout main
$ git merge fix-detached -Xtheirs
$ git branch -d fix-detached

Updating NPM Packages

Updating NPM Packages

  1. Navigate to the root directory of your project and ensure it contains a package.json file:
    • cd /path/to/project
  2. In your project root directory, run the outdated command to view outdated packages:
    • npm outdated
  3. In your project root directory, run the update command:
    • npm update


The way the wanted version is set in the package.json file may affect how updates are decided.

Version numbers are declared as a [major, minor, patch] tuple.

Caret Ranges ^1.2.3

Allows changes that do not modify the left-most non-zero element of the version specification. So 1.3.1 will update to 1.3.2 or 1.4.0. Though, 0.3.0 will update to 0.3.1 but not update to 0.4.0.

Version will update if:

  • ^1.2.3 : 1.2.3 < version < 2.0.0
  • ^0.3.0 : 0.3.0 < version < 0.4.0


Allows ‘X’, ‘x’, or ‘*’ to stand in for a number in the version specification.

Version will update if:

  • “” : any version
    • same as “*” or “x.x.x”
  • 1.x.x : 1.0.0 < version < 2.0.0
    • same as “1”
  • 1.2.x : 1.2.0 < version < 1.3.0
    • same as “1.2”