How to override a JavaScript function

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I'm trying to override a built in parseFloat function in JavaScript.

How would I go about doing that?

var origParseFloat = parseFloat;
parseFloat = function(str) {
     alert("And I'm in your floats!");
     return origParseFloat(str);
}

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You can override any built-in function by just re-declaring it.

parseFloat = function(a){
  alert(a)
};

Now parseFloat(3) will alert 3.

How to override JavaScript and jQuery functions and events?, override function. var origHelloWorld = window. helloWorld; window. helloWorld = function(argument) { origHelloWorld(argument); // after your code if you want to call original function. Types of Overriding in JavaScript 1) The First Behaviour The first way is the one we saw above when we defined a method to override the default Date 2) The Second Behaviour The second way is when we try to overload functions in JavaScript. Remember, JavaScript does not 3) The Third Behaviour

You could override it or preferably extend it's implementation like this

parseFloat = (function(_super) {
    return function() {
        // Extend it to log the value for example that is passed
        console.log(arguments[0]);
        // Or override it by always subtracting 1 for example
        arguments[0] = arguments[0] - 1;
        return _super.apply(this, arguments);
    };         

})(parseFloat);

And call it as you would normally call it:

var result = parseFloat(1.345); // It should log the value 1.345 but get the value 0.345

Overriding JavaScript functions, In this video we will discuss how to override a JavaScript function. This is continuation to Part 60 Duration: 7:18 Posted: Feb 10, 2015 Introduction It is true that JavaScript supports overriding, not overloading. When you define multiple functions that have the same name, the last one defined will override all the previously defined ones and every time when you invoke a function, the last defined one will get executed. The following example overrides the user-defined function.

You can do it like this:

alert(parseFloat("1.1531531414")); // alerts the float
parseFloat = function(input) { return 1; };
alert(parseFloat("1.1531531414")); // alerts '1'

Check out a working example here: http://jsfiddle.net/LtjzW/1/

Method Overriding In JavaScript, It is true that JavaScript supports overriding, not overloading. When you define multiple functions that have the same name, the last one defined  Override Event. Below code snippet show that with the help of .off () function we can override events in jquery. $ (document).off ('event', 'selector').on ('event', 'selector', function () { // Your code }); 1. 2. 3. $(document).off('event', 'selector').on('event', 'selector', function() {. // Your code. });

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Prevent Overriding Javascript Function, Let's see another similar example. This time, we would override the alert functionality. The default behavior of the alert function in JavaScript is to display a small  Overriding “toString()” Method. Overriding the “toString()” method is simple, we need to define the function “toString()” in the object that we are creating.

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Comments
  • i wouldn't recommend doing that. perhaps create another function within the prototype?
  • You could just do function parseFloat() {}, but there is as far as I know no way to actually call the native version then.
  • Overriding function is very common practice.
  • Is it also possible to call the native version inside a custom parseFloat? I tried this but it errors 'Maximum call stack size exceeded': jsfiddle.net/huZG2. Thanks.
  • Your string is broken :)
  • @David Waters: I'm afraid your current code alerts forever in Chrome.
  • @David Keep in mind that function are hoisted, which mean that origParseFloat points to the function you declare right after. This would work.
  • @HoLyVieR thanks Fixed by assigning function to name answer updated see jsfiddle.net/huZG2/6
  • But there is no concept of function overriding in this answer, i.e. overriding functionality but keeping the function name same.