Omg the cookie is being changed, but where? Give me a breakpoint when JS changes my cookies!

breakOn(document, 'cookie');

Some JS is getting the scrollTop value causing massive Recalculate Styles costs.. Who is the perpetrator PERFetrator?

breakOn(document.body,'scrollTop', 'read')

This works really well as a snippet in the Chrome DevTools:

By default, breakOn will only break when JS attempts to change the value of a property. The third optional argument takes 'read' if you'd also like to break when values are read.

It is also possible to disable/enable a breakpoint by using methods provided by returned object.

var bp = breakOn(document, 'cookie');
// found it!
bp.disable();
// something else is up....
bp.enable();

Dave Methvin, Paul Irish, fat, and these handsome people

• @fat blogged "debugging javascript" with an idea (attributed to @mracus) I'd talked about but hadn't seen yet

Object.defineProperty(twttr, 'profile', {
    get: function () {
      debugger
    }
  , set: function (val) {
      debugger
    }
})

• I asked for help generalizing it and saving the initial value
• Dave Methvin answered the call
• 18 months later I forked it, made this repo, and tried to ship it finally.