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A toy instrumentation for profiling JavaScript code
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esprof is a dynamic analysis tool for JavaScript. It transforms a JavaScript program into a semantically equivalent (well, almost equivalent) JavaScript program with hooks inserted to trigger events during the execution. Custom profilers can then be written to take intercept these events.


esprof is not (yet) available via npm, so installation is currently manual :

$ git clone
$ cd esprof
$ npm install

Instrumenting the code

Code can be instrumented using the esprof script located in the bin directory :

$ bin/esprof myprog.js > myprog_instrumented.js

The previous command will instrument the code to generate all events. More fined grained control can be achieved by specifying a specific profiler to use:

$ bin/esprof --profiler runtime/tracer.js myprog.js > myprog_instrumented.js

In this case, esprof will infer the events required by the profiler, and instrument the code for this particular set of events. Alternatively, events can be disabled with switches:

$ bin/esprof --no-functionDefined --no-call myprog.js > myprog_instrumented.js

Running the instrumented code

The instrumented code requires the runtime.js library to execute. This file is located in the runtime directory. For example, using d8 :

$ d8 runtime/runtime.js myprog_instrumented.js

Running the builtin profilers

Two profilers are provided in the runtime directory: tracer.js and profiler.js. To use a profiler, simply load it after runtime.js, e.g. :

$ d8 runtime/runtime.js runtime/tracer.js myprog_instrumented.js

To produce the profiling output when using profiler.js, simply load profiler_end.js at the end of the execution, or insert a call to esprof$dumpProfile(); in the code at a point when output is required.

Writing profilers using esprof

Custom profilers can be written easily by registering callbacks for one or more events:

esprof.on("methodEntry", function (event, fn, args, loc) {

esprof.on("methodExit", function (event, fn, retval, loc) {

esprof.on("alloc", function (event, obj, kind, loc) {

Supported events (and their associated callback signatures) are :

  • methodEntry : (event, fn, args, loc)

    Called when a function starts executing

  • methodExit : (event, fn, retval, loc)

    Called when a function returns normally

  • alloc : (event, obj, kind, loc)

    Called when an object is allocated

  • propRead : (event, obj, prop, loc)

    Called when a property is read from an object

  • propWrite : (event, obj, prop, val, loc)

    Called when a property is written to an object

  • beforeCall : (event, recv, fn, args, loc)

    Called before a function call (on the caller side)

  • afterCall : (event, recv, fn, args, loc)

    Called after a function call returns normally (on the caller side)

  • functionDefined : (event, fn, loc)

    Called when a function is defined (declared or created as a result of a function expression)

Current limitations

  • esprof does not currently handle code that is evaluated dynamically, e.g. through eval or the Function constructor
  • esprof does not detect object creations that happen inside of the JavaScript standard library (e.g. Array.splice). However, profilers can intercept calls and inspect return values to achieve this if required.
  • Property accesses resulting through with statements are not handled.
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