Advanced Frontend Debugging

Panos Astithas / @pastith

About me

Firefox team photo
  • Member of the Firefox Developer Tools team
  • Mainly working on the JS debugger and perf tools
  • Worked with JavaScript, Java, C/C++, Perl and more in 15+ yrs (seen lots of tools)


  1. The case for tools
  2. JavaScript Debugging
  3. Profiling
  4. Execution Tracing
  5. WebGL Debugging
  6. Debugging mobile apps

HTML5 & Tools

“I think the biggest mistake that we made, as a company, is betting too much on HTML5”
-- Mark Zuckerberg, Facebook CEO
“The biggest issue for HTML5 is the maturity of tools.”
-- Robert Shilston, FT Labs Director

The case for tools

  • JS has been around forever
  • People learned to code and debug using alert()
  • With better tools we can create more robust and more complex programs
  • A modern platform is not viable without great tool support


JavaScript Debugging

JavaScript Debugger

Inspecting State

  • Call stack visualization
  • Variable inspection
  • Closure inspection
  • Watch expressions
  • Inspecting from the Console

Pausing Execution

  • Breakpoints
  • Conditional breakpoints
  • Error breakpoints
  • Event breakpoints

Working with frameworks


& Performance

CPU Profiling


CPU Profiling (cont.)

  • console.profile/profileEnd
  • Concentrate on the hot spots
  • Observe self time vs. other time
  • Identify jank
  • Enable platform profiling for further insights

CPU Profiling (cont.)

Next-gen Profiler

Exposing the rendering engine behavior

Minimizing engine work

Paint flashing


Avoiding reflows

Reflow logging
  • Reflow events
  • Frame rate visualization
    • Set layers.acceleration.draw-fps: true

Minimizing engine work (cont.)

Layer border painting

  • Layer border painting
  • Set layers.draw-borders: true

Optimizing JS objects

  • Keep object shapes static
  • delete obj.prop; // bad
  • obj.prop = undefined; // good
  • obj.prop = null; // good
  • Cautiously consider Object.freeze

Memory Profiling

Memory monitor

Memory monitor


Execution Tracing

  • Tracing profiler or tracing debugger
  • Observe runtime code paths
  • Reason about program behavior
  • Understand how 3rd-party code works
  • Unsuitable for performance profiling due to overhead

Execution Tracing Implementations


Other implementations: spy-js, traceGL

Old-School Debugging

	0 (anonymous) foo.js:100
	1 (anonymous) foo.js:45
	2 (anonymous) bar.js:12
	3 (anonymous) baz.js:123
	4 main        baz.js:44
  • "Always use named functions"
  • "Some frameworks and tools understand fn.displayName"

Engines can infer function names

Anonymous functions no longer need to be named for debugging

	function f() {}          // display name: f (the given name)
	var g = function () {};  // display name: g
	o.p = function () {};    // display name: o.p
	var q = {
	  r: function () {}      // display name: q.r
	function h() {
	  var i = function() {};    // display name: h/i
	  f(function () {});        // display name: h/<
	var s = f(function () {});  // display name: s<

Modern Debugging

	0 f<          foo.js:100
	1 foo/<       foo.js:45
	2 bar/helper  bar.js:12
	3      baz.js:123
	4 main        baz.js:44
  • "Never use named functions"
  • "Don't bother with fn.displayName"


Debugging WebGL

Your browser doesn't appear to support the HTML5 <canvas> element.

WebGL demo

More demos and docs

Mobile Apps

Responsive Design

Responsive Design Mode

Test layout behavior in different screen sizes


Firefox OS Simulator

Debug issues related to APIs unavailable on desktop

Remote debugging

Remote debugging

Diagnose problems that only manifest themselves in the real thing

Extending your tools

Command line

GCLI commands (docs)

	  name: 'countdivs',
	  exec: function(args, context) {
	    return context.environment.document.querySelectorAll('div').length;

Extending your tools (cont.)

Ember Inspector

In Conclusion

  • What is already there:
    • Tools for debugging JavaScript
    • Profiling and performance tools
    • Tools for working with WebGL
    • Tools for debugging mobile apps
  • What is coming:
    • Execution tracing
    • Memory profiling tools
    • Much more!

Thank You