how do I override appendChild()?

Question

appendChild = function(message) {
    console.log("intercepted!");
}

using the code above does not seem to work.

Anyone knows?

Answer 1

What you might want to replace is Element.prototype.appendChild but it's probably a bad idea.

This example adds the text intercepted in the inserted element :

var f = Element.prototype.appendChild;
Element.prototype.appendChild = function(){f.apply(this, arguments);arguments[0].innerHTML="!Intercepted!"; };
document.body.appendChild(document.createElement("div"));

Demonstration

Answer 2

It is ill advised to overwrite native functions, but if you do it than make sure you return the appended element as well to prevent problems with code that uses the returned value of the native "appendChild" function:

window.callbackFunc = function(elem, args) {
  // write some logic here
}
window.f = Element.prototype.appendChild;
Element.prototype.appendChild = function() {
    window.callbackFunc.call(this, arguments);
    return window.f.apply(this, arguments);
};