How to check ES6 code coverage via istanbul and mocha

As Wikipedia said, code coverage is a measure used to describe the degree to which the source code of a program is executed when a particular test suite runs. Low code coverage means you haven’t tested your code thoroughly. In javascript, you can use install istanbul to check it, but problems come again when you try to run it against ES6 code. Either syntax error for ES6 code or No coverage information was collected, I finally found an relative elegant way to solve.

1. What I tried.

I use node v4.7.0. And I installed istanbul via yarn add istanbul -D, npm install istanbul --save-dev is ok as well. The version installed is 0.4.5. I tried following commands:

  • ./node_modules/.bin/istanbul cover ./node_modules/.bin/mocha
    • Result: syntax error, it’s ok since we don’t declare the transpile
  • ./node_modules/.bin/istanbul cover ./node_modules/.bin/_mocha
    • Result: as before.
  • istanbul cover ./node_modules/.bin/_mocha test/*.js --require babel-register
    • Result: syntax error, wow!
  • istanbul cover ./node_modules/.bin/_mocha test/*.js -- --require babel-register
    • Result: No error but No coverage information was collected

1.1 mocha and _mocha

Some post online will say that using _mocha solved the issue, but I tried with no luck, the difference is that _mocha is the actural execute file, mocha is just a wrapper around it.

2. Find the latest version of istanbul

Using the one of the following command to check:

  • npm info istanbul versions
  • yarn info istanbul versions

The result is very long,

'1.1.0-alpha.1' ]

But, we found the latest version, using alpha is not recommend, but it solved the problem.

3. Remove old and install the latest

  • npm uninstall istanbul
  • npm install istanbul@1.1.0-alpha.1 --save-dev


  • yarn remove istanbul
  • yarn add istanbul@1.1.0-alpha.1 --dev

4. Create your report

  • ./node_modules/.bin/istanbul cover ./node_modules/.bin/_mocha test/*.js -- --require babel-register"

Use _mocha here rather than mocha!

Now you get it:

about the error throwing case
✓ should throw an error when get a negative number
✓ should pass the throw-error test
20 passing (18ms)
======= Coverage summary =======
Statements : 94.29% ( 33/35 )
Branches : 93.75% ( 15/16 )
Functions : 100% ( 2/2 )
Lines : 93.75% ( 30/32 )

5. What does it mean?

  • Statements: How many of the statements in you code are executed.
    • Using the above as an example: 94.29% of statements get running when testing, and the number is 33, the overall number is 35.
    • Which means you need to write more tests to cover the missing 2 statements in order to achieve 100%.
  • Branches: Conditional statements create branches of code which may not be executed (e.g. if/else). This metric tells you how many of your branches have been executed.
  • Functions: The proportion of the functions you have defined which have been called.
  • Lines: The proportion of lines of code which have been executed.
    The above description comes from here. Click the link to learn a little more detail about istanbul.

6. Check the result via nice HTML

  • [OS X] open ./coverage/lcov-report/index.html
  • [Windows] start chrome /coverage/lcov-report/index.html

Simply click the file name in the result to see which line you are missing by reading the red line. The symbol E will tell you which branch you are missing to coverage.