JavaScript Variables - Instantiation using an IIFE

IIFE (Immediately-Invoked Function Expression), is a function that executes immediately after it is defined.

There are several places we can use an IIFE, but the most common is when we want to create a scope in JavaScript so that the variables defined within the function do not pollute the global scope (variables defined in the window).

Example:

(function () {
  'use strict'

  var hello = 'iS2JS'
  console.log(hello) // 'I love JavaScript'
})()

console.log(hello) // ReferenceError: hello is not defined

Let’s see how a variable can receive a value from an IIFE:

var count = (function () {
  var num = 0;

  return function () {
    return num ++;
  }
})();

count() // 0
count() // 1
count() // 2
count() // 3
count() // 4

How to pass one variable per parameter to the IIFE:

In the second parentheses, where we invoke the function, we can pass any parameter as if it were any other function.

(function (msg) {
  console.log(msg)
})("hi there what's up")

// Log 'hi there what's up'

These variables sent will be treated as scope variables of that function and will only be accessible within it, which allows IIFE to use variables in a script without interfering with others.