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A performance monitoring require hook for Node.js.

What does it do?

When you enable piano, all javascript processed by require will be processed by piano, which will wrap all function declarations in a __decl call. Whenever a piano-wrapped function is called, piano will record the length of time it took for that call to complete.

When you're done, just ask piano for the results, and you can perform analytics on the collected data.


var piano = require('piano')
  , ready = piano()

var plate   = require('plate')
  , tpl     = new plate.Template('{% for i in x %}{{ i }}{% endfor %}')
  , i       = 0
  , ctxt    = {x:[1,2,3,4,5,6,7,8,9,10]}
  , SAMPLES = 100
  tpl.render(ctxt, function(err, data) {
    if(i++ < SAMPLES)
      tpl.render(ctxt, arguments.callee)
    else {
      // let's look at some results!
      ready(function(items) {
          .filter(function(tap) { return tap.calls.length > 0; })
          .map(function(tap) {
              tap.filename.replace(process.cwd(), '.')+':'+tap.line,


var piano = require('piano')

var ready = piano([pathFragment | regex])

Hooks piano onto require.extensions['.js']. All files required after this point will be parsed by piano if they match the path fragment (or regex). If no path or regex is provided, it will attempt to process all javascript files will be processed.

ready(function(taps, helpers) { })

Call this function when you're finished running the code to be analyzed. It will call the provided callback with a list of Tap objects representing all processed functions, as well as a helpers object, that contains several 'sort' functions to help you sort your output.

Helpers include:

helpers.calls   // sort by number of calls
helpers.min     // sort by the minimum call time
helpers.max     // sort by the maximum call time
helpers.avg     // sort by the average call time   // sort by the total amount of time spent in a function

// usage:

var ready = piano();
ready(function(taps, helpers) {
        forEach(function(tap) {

ready.on('tap', function(tap, executionContext) {})

Emitted whenever a 'tapped' function is run -- that is, any function processed by piano. It will be called with two arguments -- a Tap object that represents piano's bookkeeping on the current function, and an ExecutionContext that represents all functions processed by piano, as well as the current stack depth.


Restores the original require.extensions['.js'] callback.


An execution context may only be accessed through the on('tap') event, and has the following attributes:

        // a list of all `Tap`'d functions.

        // a hash of all 'guids' for this context, with references to indices within `executionContext.functions`.

        // an integer representing the current stack depth. not 100% guaranteed to be accurate.


A tap object contains all bookkeeping done for a function processed by piano. It is available via the on('tap') function, as well as when calling ready. It has the following methods:


Returns the quickest call to the tapped function as an integer representing microseconds.


Returns the slowest call to the tapped function as an integer representing microseconds.


Return the average time taken by a function over all calls as a float representing microseconds.


Return the total number of microseconds taken by a function over the duration of the test.


Returns the original line of code that generated the tap as a string.

Tap also has the following properties:


The original function object.


An array of {start:integer, end:integer} pairs, each representing a separate call to the function (and subsequent exit of the function).


The filename the Tap was generated from.


The line number the Tap was generated from.

A Quick Note

Piano creates Taps keyed by the function content concatenated to the filename and line number that the function was seen at.

This means that Piano will not create multiple taps for inlined functions -- even though inlined functions strictly represent separate objects in V8. An example:

    var fn = function() {
        // an inlined function:
        return function() {



Will create one Tap for fn, and one Tap for the function returned by fn. If, hypothetically, piano created Taps based on whether the function was present in a list of functions, the previous code would have generated 3 taps -- one for fn, one for the first time fn()() was called, and one for the second time fn()() was called. Since this isn't very useful behavior, I opted to make Piano pay attention to the originating source of the function, not the function object itself.



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