Get all user defined window properties?

You could also compare the window against a clean version of the window instead of trying to snapshot during runtime to compare against. I ran this in console but, you could turn it into a function.

// make sure it doesn't count my own properties
(function () {
    var results, currentWindow,
    // create an iframe and append to body to load a clean window object
    iframe = document.createElement('iframe');
    iframe.style.display = 'none';
    document.body.appendChild(iframe);
    // get the current list of properties on window
    currentWindow = Object.getOwnPropertyNames(window);
    // filter the list against the properties that exist in the clean window
    results = currentWindow.filter(function(prop) {
        return !iframe.contentWindow.hasOwnProperty(prop);
    });
    // log an array of properties that are different
    console.log(results);
    document.body.removeChild(iframe);
}());

You would need to do the work for yourself. Read in all properties, on the first possible time you can. From that point on, you can compare the property list with your static one.

var globalProps = [ ];

function readGlobalProps() {
    globalProps = Object.getOwnPropertyNames( window );
}

function findNewEntries() {
    var currentPropList = Object.getOwnPropertyNames( window );

    return currentPropList.filter( findDuplicate );

    function findDuplicate( propName ) {
        return globalProps.indexOf( propName ) === -1;
    }
}

So now, we could go like

// on init
readGlobalProps();  // store current properties on global object

and later

window.foobar = 42;

findNewEntries(); // returns an array of new properties, in this case ['foobar']

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Of course, the caveat here is, that you can only "freeze" the global property list at the time where your script is able to call it the earliest time.