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What is an Immediately Invoked Function in JavaScript?

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An Immediately Invoked Function ( known as IIFE and pronounced as IIFY) is a function that runs as soon as it is defined.

Syntax of IIFE :

(function(){ 

  // Do something;

})();

To understand IIFE, we need to understand the two sets of parentheses which are added while creating an IIFE :

First set of parenthesis:

(function (){

   //Do something;

})

While executing javascript code, whenever the compiler sees the word “function”, it assumes that we are declaring a function in the code. Therefore, if we do not use the first set of parentheses, the compiler throws an error because it thinks we are declaring a function, and by the syntax of declaring a function, a function should always have a name.

function() {

  //Do something;

}

// Compiler gives an error since the syntax of declaring a function is wrong in the code above.

To remove this error, we add the first set of parenthesis that tells the compiler that the function is not a function declaration, instead, it’s a function expression.

Second set of parenthesis:

(function (){

  //Do something;

})();

From the definition of an IIFE, we know that our code should run as soon as it is defined. A function runs only when it is invoked. If we do not invoke the function, the function declaration is returned:

(function (){

  // Do something;

})

// Returns the function declaration

Therefore to invoke the function, we use the second set of parenthesis. .

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