ES6 Variables - 3

An immediately-invoked function expression (or IIFE, pronounced "iffy") is a JavaScript programming language idiom which produces a lexical scope using JavaScript's function scoping.

Runs itself immediately.

var name=“Arjun”;

//but Window has a name attribute too.

So we use IIFE

function(){

var name=“Arjun”;

}();

Let and const don’t need IIFE

{

const name=“Arjun”; //name is block scoped

}

for loop leaks its index variable.

for(var i=0;i<10;i++)

{

console.log(i); //prints 0 through 9

}

console.log(i); //prints 10 in global scope

If we have a time sensitive function in a for loop, the results may get skewed, unless we used an IIFE.

for(var i=0;i<10;i++)

{

setTimeout(function(){console.log(i);},1000); 

}

//prints 10, 10 times because in 1 sec the counter will have already run.

//replacing var with let prints out 0-9 because it acts like an IIFE.

IIFE

IIFE executes the function like it was assigned to a variable and run. It creates a function expression for the function and executes it. 

function test(){var name="Al";};
test();
//is the same thing as
(function(){var name="Al";})();
//The only difference is that the second one is executed instantly
//and the first one is executed when its name is called

IIFE Syntax

(function(){})();//is IIFE

!function(){}(); //is IIFE and returns true

-function(){}(); //is IIFE and returns NaN

$function(){}(); //is IIFE but requires JQuery

(function(){}();) //is IIFE

*IIFE produces lexical scope

*let and const behave like IIFE

*for loop does not leak