8) What is IIFE in JavaScript

IIFE: Immediately Invoked Function Expression

It is a JavaScript Function that runs as an when it is defined.

Syntax of IIFE

Let's talk about the two parenthesis in the above syntax.

( function() {} )

Outer parenthesis '()'
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.

So, instead of getting error, we have to use the first set of parenthesis that tells the compiler that this function is not the function declaration but it's function expression.

( function() {}) ();

Right-end side parenthesis '()'
So, IIFE states that the function should invoke immediately as soon as it is defined.
And as we know to run a function we need to invoke it.
If we don't invoke it, function declaration is returned.
That's why this second parenthesis is just for invoking.