javascript - Detect if function is native to browser

Question

I am trying to iterate over all the globals defined in a website, but in doing so I am also getting the native browser functions.

var numf=0; var nump=0; var numo=0; 
for(var p in this) { 
    if(typeof(this[p]) === "function"){
        numf+=1;
        console.log(p+"()");
    } else if(typeof p != 'undefined'){
        nump+=1;
        console.log(p);
    } else { 
        numo+=1;
        console.log(p);
    }
}

Is there a way to determine if a function is native to the browser or created in a script?

Answer 1

You can call the inherited .toString() function on the methods and check the outcome. Native methods will have a block like [native code].

if( this[p].toString().indexOf('[native code]') > -1 ) {
    // yep, native in the browser
}

---

Update because a lot of commentators want some clarification and people really have a requirement for such a detection. To make this check really save, we should probably use a line line this:

if( /{s+[native code]/.test( Function.prototype.toString.call( this[ p ] ) ) ) {
    // yep, native
}

Now we're using the .toString method from the prototype of Function which makes it very unlikely if not impossible some other script has overwritten the toString method. Secondly we're checking with a regular expression so we can't get fooled by comments within the function body.