ES6 Immediately-invoked function expression

   ES6 Immediately-invoked function expression

IIFE was one of the most used patterns in the ES5 standard, as functions were the only way to declare a scoped block of code. So the following code will result with a ReferenceError:

(function() {
    var scoped = 42;
}());

console.log(scoped); // ReferenceError

ECMAScript introduced the block scoped let and const, so instead of using an IIFE, we can have the following code:

{
    let scoped = 42;
}

console.log(scoped); // ReferenceError

The later example is way more clear and understandable.

IIFEs were used as a building block in the revealing module pattern. Its advantage is that it clearly separates between what is public and what is private:

var myModule = (function() {
    // private variable, accessible only inside the IIFE
    var counter = 0;

    function increment() {
        counter++;
    }

    // publicly exposed logic
    return {    
        increment: increment
    }
}());

Instead of using this pattern, with ES6 we can use modules. Modules declare their own scope and variables created inside the module will not polute the global object:

// myModule.js

let counter = 0;

export function increment() {
    counter++;
}    

// logic.js

import {increment} from 'myModule.js';

increment();

The only case, where you may want to use an IIFE is with an immediately-invoked arrow functions. Sometimes you only can produce a result via a sequence of statements, not via a single expression and if you want to inline these statements, you have to immediately invoke a function:

const SENTENCE = 'Hello world, how are you?';
const REVERSE = (() => {
    const array  = [...SENTENCE];
    array.reverse();
    return arr.join('');
})();

Keep in mind, that you need to wrap the arrow function in parentheses (they doesn't wrap the whole function).

As a rule of thumb you want to avoid using IIFEs in ES6, as there are other way to handle global variable polution.

https://hashnode.com/post/do-es6-modules-make-the-case-of-iifes-obsolete-civ96wet80scqgc538un20es0

https://ultimatecourses.com/blog/everything-you-wanted-to-know-about-javascript-scope