Usage

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();

breakOn also supports Conditional Breakpoints when the 4th argument is a function. Say you know a property is being changed after the 4th change.

    var i = 0;
    var bp = breakOn(document, 'cookie', false, function(v) {
        return i++ >= 4;
    });

Or something is leaving a property undefined

    var bp = breakOn(document, 'cookie', false, function(v) {
        return typeof v === 'undefined';
    });

Authors

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

Alternatives

• javascript-breakpoint-collection by @mattzeunert. Find what code is causing a browser behavior. Very nice.
• debug_utils by @amasad has similar functionality, with a larger scope

History

• @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.