Question

I would like to override a Javascript built-in function with a new version that calls the original (similarly to overriding a method on a class with a version that calls super in many languages). How can I do this?

For example…

window.alert = function(str) {
    //do something additional
    if(console) console.log(str);

    //super.alert(str) // How do I do this bit?
}

Answer 1

Store a reference to the original function in a variable:

(function() {
    var _alert = window.alert;                   // <-- Reference
    window.alert = function(str) {
        // do something additional
        if(console) console.log(str);
        //return _alert.apply(this, arguments);  // <-- The universal method
        _alert(str);                             // Suits for this case
    };
})();

The universal way is <original_func_reference>.apply(this, arguments) – To preserve context and pass all arguments. Usually, the return value of the original method should also be returned.

However, it’s known that alert is a void function, takes only one argument, and does not use the this object. So, _alert(str) is sufficient in this case.

Note: IE <= 8 throws an error if you try to overwrite alert, so make sure that you’re using window.alert = ... instead of alert = ....