How to return window.performance object to CasperJS scope

Question

I'm trying to return window.performance object from the web page back to casper's scope with the following code but I'm getting null. Can someone explain why?

    performance = casper.evaluate ->
        return window.performance
    @echo performance

Answer 1

PhantomJS 1.x doesn't implement window.performance, so you can't use it.

PhantomJS 2.0.0 implements it, but it doesn't implement the window.performance.toJSON() function. The problem with PhantomJS is that you have to access this information through evaluate(), but it has the following limitation:

Note: The arguments and the return value to the evaluate function must be a simple primitive object. The rule of thumb: if it can be serialized via JSON, then it is fine.

Closures, functions, DOM nodes, etc. will not work!

You will have to find your own way of serializing this in the page context and passing it to the outside (JavaScript):

var performance = casper.evaluate(function(){
    var t = window.performance.timing;
    var n = window.performance.navigation;
    return {
        timing: {
            connectStart: t.connectStart,
            connectEnd: t.connectEnd,
            ...
        },
        navigation: {
            type: n.type,
            redirectCount: n.redirectCount
        },
        ...
    };
});

or look for a deep copy algorithm that produces a serializable object (from here):

var perf = casper.evaluate(function(){
    function cloneObject(obj) {
        var clone = {};
        for(var i in obj) {
            if(typeof(obj[i])=="object" && obj[i] != null)
                clone[i] = cloneObject(obj[i]);
            else
                clone[i] = obj[i];
        }
        return clone;
    }
    return cloneObject(window.performance);
});
console.log(JSON.stringify(perf, undefined, 4));