Overwriting native console.log function in JavaScript

Question

I want to override the console.log function in order to save the data other scripts log. My script executes before any other script, so I have full control over the environment. This is my current function for overwriting console.log:

+function(){
    var a = Function.prototype, b = 'toString', c = a[b], d = c.apply(c).split(c.name), e = [], f = [], g = function(a, b){
        e.push(a);
        f.push(b);
        return a;
    }, _ = [];

    a[b] = function(h){
        return g(function toString(){
            var i = e.indexOf(this);
            return i != -1 ? f[i] || d.join(this.name) : h.apply(this);
        });
    }(c);

    console.log = function(a){
        return g(function log(){
            var b = [], i;
            for(i in arguments) b.push(arguments[i]);
                _.push(b.slice());
            return a.apply(this, arguments);
        });
    }(console.log);
}();

My question is simple: is it possible that some script executed after this can reveal that I have an overwritten console.log function?

I will use it for debugging scripts sent from users to my Node.js server application. I am using vm module, so I need to overwrite sandbox the object's console.log function in order to trace what users log to console in received scripts.

If this code looks obfuscated, here is a better version.

Answer 1

At first sight, this is really clever, because inspecting console.log will still show log() { [native code] }

But then, what is this property prototype that is not present in the original console.log?

Even more interesting, in Chrome Canary, console.log.prototype.constructor will show the location of the implementation, which is clearly in user space:

Object
  constructor:log()
    arguments:null
    caller:null
    length:0
    name:"log"
    prototype:Object
    __proto__:()
    [[FunctionLocation]]:VM1191:16
    [[Scopes]]:Scopes[3]
  __proto__:Object

In node.js, the constructor just behaves weirdly and is indicative of weird things going on.

Furthermore, since this is codereview, the commonly accepted practice for immediately self executing is

(function functionName(arguments){ 
})();

That + is super hokey, and will be used in future codegolfing entries ;)

Answer 2

I tried hacking around this in various ways and practically the only way I could detect it was wrapped was by having an old reference to console.log and using ===. Since you state your code will run before any other scripts will run, this shouldn't be a problem.

That said, there's also console.info, warn, trace, so maybe you'd want to wrap those too?

From a code quality perspective, your variable naming needs work. You can haul it through a minifier later; what you have right now is unmaintainable. I don't even know what half of it does thanks to the variable naming.

Answer 3

Because my original code looks very obfuscated and this article says that OP is not allowed to edit code after accepting an answer, I am writing here beautified and easy-to-understand code:

+function(){
    var FunctionPrototype = Function.prototype;
    var toStr = 'toString';
    var originalFunc = FunctionPrototype[toStr];
    var namePattern = originalFunc.apply(originalFunc).split(originalFunc.name);
    var overwritenFuncs = [];
    var overwritenFuncsNames = [];
    var overwrite = function(originalFunction, overridenToStr){
        overwritenFuncs.push(originalFunction);
        overwritenFuncsNames.push(overridenToStr);
        return originalFunction;
    };
    var tracedLogs = [];

    FunctionPrototype[toStr] = function(h){
        return overwrite(function toString(){
            var i = overwritenFuncs.indexOf(this);
            return i != -1 ? overwritenFuncsNames[i] || namePattern.join(this.name) : h.apply(this);
        });
    }(originalFunc);

    console.log = function(originalConsoleLog){
        return overwrite(function log(){
            var b = [], i;
            for(i in arguments) b.push(arguments[i]);
            tracedLogs.push(b.slice());
            return originalConsoleLog.apply(this, arguments);
        });
    }(console.log);
}();