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How to prevent overriding using Immediately Invoked Function Expression in JavaScript ?
  • Last Updated : 01 Jul, 2020

Overriding is basically when you define multiple functions or variables 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. Overriding usually happens when you have multiple javascript files in your page. It can be an external file or the file which you have created internally. This usually occurs due to the presence of variables or functions with the same name in the global scope.

Suppose we have the following files:
Filename: index.html




<!DOCTYPE html>
<html>
<head>
    <title>IIFE</title>
    <script src="script1.js"></script>
    <script src="script2.js"></script>
    <script src="app.js"></script>
</head>
<body></body>
</html>

This file has linked script1.js, script2.js and app.js as external javascript files which are given below:

Filename: script1.js




var arsalanGreeter = {};
arsalanGreeter.name = "Arsalan";
var greeting = "Hello ";
arsalanGreeter.sayHello = function() {
    console.log(greeting + arsalanGreeter.name);
}

Filename: script2.js






var johnGreeter = {}
johnGreeter.name = "John";
var greeting = "Hi ";
johnGreeter.sayHi = function() {
    console.log(greeting + johnGreeter.name);
}

Filename: app.js




arsalanGreeter.sayHello();
johnGreeter.sayHi();

Here app.js is responsible for invoking functions inside script1.js and script2.js and you will get below output in your console window as shown below:

Well, this happens because we have the same variable name in script1.js and script2.js as “greeting”. This is the reason script2.js is overriding over script1.js.

In order to fix this issue, we will use the concept of Immediately Invoked Function Expression (IIFE) as described below:

Now our files will look like this:
Filename: index.html




<!DOCTYPE html>
<html>
<head>
    <title>IIFE</title>
    <script src="script1.js"></script>
    <script src="script2.js"></script>
    <script src="app.js"></script>
</head>
<body></body>
</html>

Filename: script1.js




(function (window) {
   var arsalanGreeter = {};
   arsalanGreeter.name = "Arsalan";
   var greeting = "Hello ";
   arsalanGreeter.sayHello = function() {
       console.log(greeting + arsalanGreeter.name);
   }
   window.arsalanGreeter = arsalanGreeter; 
})(window);

Filename: script2.js




(function (window) {
   var johnGreeter = {};
   johnGreeter.name = "John";
   var greeting = "Hi ";
   johnGreeter.sayHi = function() {
    console.log(greeting + johnGreeter.name);
   }
   window.johnGreeter = johnGreeter; 
})(window);

In the above two files, we have created IIFE and passed to the window variable and invoked that.
Filename: app.js




arsalanGreeter.sayHello();
johnGreeter.sayHi();

After doing the above mentioned changes that is adding Immediately Invoked Function Expression, the output of the console will look as shown below:




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