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Code Coverage Done Right!


JS code coverage done right. Why?

See an example report

Get Started

# install 
npm install -g steamshovel
# instrument 
shovel mySourceDirectory myInstrumentedDirectory
# introspect 
mocha -R steamshovel

For a more detailed guide to the SteamShovel internals, see the API section.

Why Should You Use SteamShovel

"But I like Blanket/Istanbul/JScover!" I hear you say. "Why should I move?"

That's a great question. It comes down to this:

Your existing instrumentor is lying to you.

That's right. When you run your tests, and the coverage data is collected, the instrumentor only pays attention to whether a code branch was run... or wasn't. Unfortunately this is a gross oversimplification of how code is invoked by a test suite, and it will leave you with a false sense of security.

The hypothesis behind SteamShovel is this:

Your unit tests should run your code directly. Indirect invocation is probably unintentional, and that code shouldn't be considered tested.

In order to support this hypothesis, SteamShovel records the stack depth of the instrumentor at every invocation, and calculates the 'testedness' of a given branch by applying a weighted inverse logarithm to this depth.

This gives you a much more accurate code coverage value. The variance in 'coverage' as defined by SteamShovel and a conventional instrumentor like Istanbul can be as much as 50% — that's how much Istanbul is over-reporting.

What else?

SteamShovel can do a whole lot more, too:

  • Profile timing, order, and stack depth
  • Record environment data like memory or load on a per-expression level (set the environment variable REPORTER_OUTPUT to csv and you'll get an expression level dump of timing and memory data!
  • Save the result of every evaluation of every expression in your codebase, and let you step through the results of your test run, interactively. (Coming soon!)
  • SteamShovel can now auto-instrument your code as you require it! Simply set the AUTO_INSTRUMENT environment variable to true before running the SteamShovel reporter. Remember that this won't instrument code that isn't required, so the manual instrumentor may be more accurate in some cases.

You can override the SteamShovel instrumentor with your own, and record anything you might want to record! The sky is the limit.

SteamShovel's exported memory data from a test run

Seen here: SteamShovel's recorded memory data from a test run, visualised in, uh...

See also: Stack depth of calls over time, Call latency over run-time of test suite — you're only limited by your imagination here.


Because SteamShovel records so much more from your code, it does have a considerable performance impact. However, the accuracy of your test coverage data should be more important than how quickly it runs.

Also, (if you couldn't tell already!) SteamShovel is early in its development. Be careful: there will be an excruciating number of bugs.

I am most of the way through an important refactor which will make steamshovel ready for use by anyone!


  • Stats generation needs to be a lot more abstract, and a lot faster. Look forward to a huge amount of progress in this area!
  • Stats should also allow arbitrary queries to be developed against the data which could be defined by a model and included in the template.
  • Gotta make things even easier to use!
  • More info in the default HTML output.
  • Memory usage and timing output as CSV & JSON!


Processing Files

You can recursively instrument one or more directories or files by using the SteamShovel instrument processor. This is the same system the CLI tool uses to instrument your code.

var steamshovel = require("steamshovel");
steamshovel.process("./lib", "./lib-instrumented")
    .on("complete", function(inFile) {
        console.log("The instrumentation of %s is complete!", inFile);

steamshovel.process ( <string | array> inFile, outFile <string | array> , EventEmitter emitter )

This function takes a string describing an input file or directory, or an array of strings of files/directories. If the first parameter is an array, the second must also be an array, and their lengths must be the same.

The function returns an event emitter, to which it emits the following events:

  • complete (file) — the processing for one of the directly specified files or directories is complete.
  • dircomplete (dir) — the processing for a given directory is complete (this is emitted for directories that SteamShovel discovers, too — not just for directly specified trees.)
  • error (error, path/file) — an error was emitted from a filesystem operation.
  • ignore (file) — this file was explicitly ignored (the directive steamShovel:ignore was found in the file.)
  • instrumenterror (err, file) — an error occurred while attempting to instrument a given file (most likely a syntax error.)
  • mkdir (path) — emitted when SteamShovel creates a new directory
  • nojs (file) — SteamShovel discovered a file, but it is not a JavaScript file and will be ignored. (It will be written into the new tree, but not instrumented.)
  • processdir (dir, out) — Emitted when SteamShovel begins processing a new directory
  • readdir (dir) — Emitted after SteamShovel reads the contents of a directory.
  • readfile (file) — Emitted after SteamShovel completes reading a file.
  • writefile (file) — Emitted after SteamShovel completes writing a file.

Instrumenting Code

You can instrument code on a file by file basis by using the steamshovel.instrument method.

steamshovel.instrument ( data, [ filename, incorporateMap ] )

This function takes a string containing the JavaScript source code and returns the transformed code with instrumentation added.

Presently, the code is instrumented by replacing all applicable expressions with SequenceExpressions (you'll probably know these as lists of expressions delimited by way of the comma operator.) These make a function call with a unique ID to the individual expression (incorporating a hash of the filename and an expression index.)

The filename is required if you want to instrument more than one file. Otherwise you could end up with ID conflicts. (This won't ever be a problem if you let SteamShovel instrument for you.)

The third boolean parameter enables you to turn off the 'map', basically a non-human-readable object definition for the expression instrumentation in the code. It is advisable that you leave this alone, as the code won't run without it, but switching it off can be useful for testing the output of the instrument function.