Tracking when / where a variable is set in Javascript using FireBug

Question

I am currently using FireBug to see how hypem delivers it the content to allow its flash player to work (specifically the play button functionality). I have found that the trackList variable is populated with the data but I do not know where it is set.

How can I track when / where this collection is modified? I can add a watch to it but don't know how to trap when it is set.

FYI I am doing this as I want some of this functionality and want to understand how they have done this.

Alexander Abramov · Accepted Answer · 2014-08-18 16:11:07Z

2

Here is how you use defineProperty to do it. In my example I'm logging each time window.$ is assigned, along with a call stack. Of course instead of logging you can trigger a debugger; or do something else.

Object.defineProperty(window, "$",
{
  set: function(val) {
    console.log("$ was set to '",val,"' at ",Error().stack);
    this.$internal=val;
  },
  get: function() { return this.$internal; }
});

answered Aug 18, 2014 at 16:11

Alexander Abramov

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Comments

SLaks · Accepted Answer · 2010-07-21 14:27:47Z

1

Unfortunately, this isn't directly possible.

Instead, you can search through the Javascript and see where the variable is set, then put a normal breakpoint on each line that you find.

answered Jul 21, 2010 at 14:27

SLaks

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3 Comments

Chris Over a year ago

I tried this but unfortunately can't seem to find out where it is set!

SLaks Over a year ago

Try calling defineProperty and putting a breakpoint or console.log call in the property.

Chris Over a year ago

Sorry but how do I use defineProperty?

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