Babel 6.x plugin to add instrument code with Istanbul-compatible `__coverage__` variable.

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A Babel plugin that instruments your code with __coverage__ variable. The resulting __coverage__ object is compatible with Istanbul, which means it can instantly be used with karma-coverage and mocha on Node.js (through nyc).
Note: This plugin does not generate any report or save any data to any file; it only adds instrumenting code to your JavaScript source code. To integrate with testing tools, please see the Integrations section.


Install it:
npm install --save-dev babel-plugin-instrument-istanbul

Add it to .babelrc in test mode:
  "env": {
    "test": {
      "plugins": [ "__coverage__" ]



It just works with Karma. First, make sure that the code is already transpiled by Babel (either using karma-babel-preprocessor, karma-webpack, or karma-browserify). Then, simply set up karma-coverage according to the docs, but don’t add the coverage preprocessor. This plugin has already instrumented your code, and Karma should pick it up automatically.
It has been tested with bemusic/bemuse project, which contains ~2400 statements.

mocha on node.js (through nyc)

Configure Mocha to transpile JavaScript code using Babel, then you can run your tests with nyc, which will collect all the coverage report. You need to configure NYC not to instrument your code by setting this in your package.json:
"nyc": {
  "include": [ "/" ]

Ignoring files

You don't want to cover your test files as this will skew your coverage results. You can configure this by configuring the plugin like so:
"plugins": [ [ "__coverage__", { "ignore": "test/" } ] ]

Where ignore is a glob pattern.
Alternatively, you can specify only which will take precedence over ignore:
"plugins": [ [ "__coverage__", { "only": "src/" } ] ]

Canned Answers

There’s already Isparta. Why another coverage tool?

Note: Some text in this section is outdated and I say this instead of keeping this section up-to-date lol. So Isparta is now unmaintained and a new version of Istanbul that supports arbitrary compile-to-JS language is coming…

Isparta is currently the de-facto tool for measuring coverage against ES6 code, which extends Istanbul with ES6 support through Babel. It works very well so far, but then I hit some walls with it.
So I’ve been trying to get webpack 2 to work with Istanbul/Isparta. To benefit from webpack 2’s with tree shaking, I need to keep import and export statements in my ES6 modules intact. However, with this setup I can’t get code coverage to work. Inspecting Isparta’s source code reveals how it works:
  1. It uses Babel to transpile ES6 back into ES5, saving the source map.
  2. It uses Istanbul to instrument the transpiled source code. This produces some initial metadata (in a global variable called __coverage__) which contains the location of each statement, branch, and function. Unfortunately, these mapings are mapped to the transpiled code. Therefore,
  3. The metadata is processed using source map obtained from step 1 to map the location in transpiled code back to the location in the original source code.
  4. The final instrumented code in ES5 is generated. This code shouldn’t be processed through Babel again, or it will be redundant and leads to slower builds.

Since transforming import/export statements has now been disabled, instrumentation now dies at step 2, because the Esprima that Istanbul is using cannot process import/export statements.
So I looked for something else, and I found babel-plugin-transform-adana. I tried it out immediately. It turns out that although adana also generates the __coverage__ variable, it uses its own format which is not compatible with most existing tools (e.g. istanbul’s reporter, karma-coverage and nyc). Tools need to be reinvented for each test harness.
So I went back to use webpack 1 for the time being.
Now, with lots of tools to help developers author Babel 6 plugins, such as the Babel Handbook and the AST Explorer, it’s not that hard to create Babel plugins today. So I gave it a shot. This is my first Babel plugin.
It turns out that I can create a rudimentary instrumenter with Babel 6 in roughly 300 LOC (compare to Istanbul’s instrumenter which has ~1,000 LOC). Babel has A LOT of cool stuff to make transpilation easy, from babel-template to babel-traverse to babel-helper-function-name. Babel’s convenient API also handles a lot of edge cases automatically. For example, if a function begins with 'use strict' statement, prepending a statement into its body will insert it after the 'use strict' statement. It also transparently converts if (a) b into if (a) { b } if I want to insert another statement before or after b.
I haven’t tested this with webpack 2 yet.

Is it stable?

Well, I wrote most of it in two nights and have only tested some basic stuffs. So speaking in terms of maturity, this one is very new.
However, I tried using this in some bigger projects, such as bemusic/bemuse (which contains around 2400 statements). It works, with only few problems (which have now been fixed), and now it works fine.
Note: If you’re using babel-plugin-\_\coverage\_\ inside a larger-scale application, feel free to send pull request and update this section kthx!

Is the resulting coverage differ from Istanbul/Isparta?

Sometimes there’s so much logic in a single statement (usually due to functional programming techniques), it is very likely that not every part of a statement will be covered by tests (see picture below). Since most coverage service only cares about statement coverage, and I want coverage numbers to be very frank, babel-plugin-instrument-istanbul will treat certain expressions as statements.
Most likely, this means if you switch to this plugin your coverage percentage will most likely go down compared to when you were using Istanbul or Isparta. But if you use a lot of arrow functions, this means your coverage will be more accurate! Here’s an example from the bemusic/bemuse project:
Here are some notable differences:
  1. Arrow function expression body is treated as a statement, because its body may never be evaluated.
   const x = a => b => c => a + b + c
// <--------------------------------> S1
//                <-----------------> S2
//                     <------------> S3
//                          <-------> S4
  1. Logical operator’s right hand side expression is treated as a statement, because it may be short-circuited.
   const value = a + b || a - b
// <--------------------------> S1
//                        <---> S2
  1. Each of conditional operator’s branch is treated as a statement because it may not be evaluated.
   const value = op === 'add' ? a + b : a - b
// <----------------------------------------> S1
//                              <---> S2
//                                      <---> S3
  1. Default parameters and values for destructuring are treated as a statement, because the default may not be evaluated.
   function setValue (newValue = defaultValue) { }
//                                             <-> S1
//                               <----------> S2

How do I ignore branches/statements?

I haven’t implemented it. I once posted an issue on Isparta asking about ignoring statements in Isparta (which is now fixed). But nowadays I just think that “coverage is just a number,” so I don’t need them anymore. If you want it, pull requests are welcome!