How to detect creation of new global variables?

Question

I want watch for the creation of new global variables in Javascript so that, anytime a global variable is created, an event is fired.

I've heard of the watch() function but that is only for watching for specific variable names. I want a catchall.

I'm writing a framework which takes any variables inserted into the global namespace and places them in a data structure for tracking. — Sridatta Thatipamala, Feb 23 '11 at 07:56
Perhaps this post will inspire some ideas http://stackoverflow.com/questions/1029241/javascript-object-watch-for-all-browsers — maerics, Feb 23 '11 at 08:00

score 6 · Answer 1 · edited May 23 '17 at 11:43

6

If you already know which names pollute your global namespace (see Intercepting global variable definition in javascript), you can use this trick to figure out when does it actually happen:

window.__defineSetter__('someGlobalVar', function() {
    debugger;
});

Be sure to have your developer tools open when you run this. Obviously works only if your browser supports __defineSetter__ but that's true for modern browsers. Also, don't forget to remove your debug code after you've finished.

Found here.

edited May 23 '17 at 11:43

Community

1
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answered Jan 11 '13 at 10:52

Dmitry Pashkevich

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This should be the approved solution – Gabriel Furstenheim Apr 27 '21 at 09:42

selbie · Accepted Answer · 2011-02-23T08:12:57.220

3

I don't know how to make this work "on demand" as soon as a var is created, but I can suggest a polling approach. In a browser window, all global become a member of the global "window" object. (Because technically, "window" is the "global object"). So you could do something like the following:

1) enumerate all the properties on a window

window.proplist = window.proplist || {};
for (propname in window) {

    if (propname !== "proplist") {
      window.proplist[propname] = true;
    }
}

2) Set a timer to periodically "poll" window for new properties

setInterval(onTimer, 1000);

3) Wake up on the timer callback and look for new props

function onTimer() {
        if (!window.proplist) {
            return;
        }

        for (propname in window) {

              if (!(window.proplist[propname])) {
                 window.proplist[propname] = true;
                 onGlobalVarCreated(propname);
              }
        }
}

edited Feb 23 '11 at 08:12

answered Feb 23 '11 at 08:03

selbie

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`foreach` isn't a valid keyword! And place the `if (window.proplist)` checks outside the `for` loop. – Raynos Feb 23 '11 at 08:06
NO FOREACH IN JAVASCRIPT! Won't give you -1, fix that. `for(var propname in window)` would work. – Cipi Feb 23 '11 at 08:06
Oops...good catch guys. Fixed above. The poster only hasn't earned enough stackoverflow points for me to actually test my answer (or optimize it!). :) – selbie Feb 23 '11 at 08:08
@Cipi https://developer.mozilla.org/en/JavaScript/Reference/Statements/For_each...in your wrong ;) – Raynos Feb 23 '11 at 08:08
@Raynos: Chrome 9.0 doesn't have "for each" nor "foreach"... where can I use that? – Cipi Feb 23 '11 at 08:12
This also doesn't handle deletion and recreation. – Raynos Feb 23 '11 at 08:12
@Cipi but JavaScript has `for each` :) – Raynos Feb 23 '11 at 08:13
While this solution works, it doesn't tell you where was the global variable created... – Dmitry Pashkevich Jan 14 '13 at 12:06

score 0 · Answer 3 · answered Feb 23 '11 at 08:22

Afaik, .watch() is only SpiderMonkey (Firefox).

I played around with a polling function, I finally came up with this:

var mywatch = (function() {
    var last = {
        count: 0,
        elems: {}
    };    

    return function _REP(cb) {
        var curr = {
                count: 0,
                elems: {}
            },
            diff = {};

        for(var prop in window) {
            if( window.hasOwnProperty(prop) ) {
                curr.elems[prop] = window[prop]; curr.count++;
            }
        }

        if( curr.count > last.count ) {
            for(var comp in curr.elems) {
                if( !(comp in last.elems) ) {
                    diff[comp] = curr.elems[comp];
                }
            }
            last.count = curr.count;
            last.elems = curr.elems;

            if(typeof cb === 'function')
                cb.apply(null, [diff]);
        }

        setTimeout(function() {
            _REP(cb);
        }, 400);
    };
}());

And then use it like:

mywatch(function(diff) {
    console.log('NEW GLOBAL(s): ', diff);
});

Be aware that this only handles new globals. But you can easily extend this for the case last.count > curr.count. That would indicate that global variables were deleted.

score 0 · Answer 4 · answered Feb 23 '11 at 08:22

You cant have an event fired when some script does var v = 10, but as selbie said, you can poll the window object... I meant to suggest the same, but he beat me to it. Here's my other example... you count how many window objects are there, and execute GlobalVarCreated() function:

var number_of_globals = 0; //last known globals count

var interval = window.setInterval(function(){

   var new_globals_count = 0; //we count again
   for(var i in window) new_globals_count++; //actual counting
   if(number_of_globals == 0) number_of_globals = new_globals_count; //first time we initialize old value
   else{
     var number_of_new_globals = new_globals_count - number_of_globals; //new - old
     if(number_of_new_globals > 0){ //if the number is higher then 0 then we have some vars
         number_of_globals = new_globals_count; 

         for(var i = 0; i<number_of_new_globals; i++) GlobalVarCreated(); //if we have 2 new vars we call handler 2 times...
     }

   }

},300); //each 300ms check is run


//Other functions
function GlobalVarCreated(){}
function StopInterval(){window.clearInterval(interval);}

You can load up that code in Chrome or FF console only change: function GlobalVarCreated(){console.log("NEW VAR CREATED");} and test it:

var a = 10

b = 10

String NEW VAR CREATED is displayed 2 times.

How to detect creation of new global variables?

4 Answers4

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