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Frontend Infrastructure: IIFE — Immediately Invoked Function Expression

• 3 min read
lettersPhoto by Fernando Rodrigues

IIFE stands for Immediately Invoked Function Expression which means it's a function expression that self execute. It is also known as a Self-Executing Anonymous Function.

To understand the concept, let's see the syntax:

(function () {
  // execute something here
  // everything created in this scope
  // will not be accessible in the global scope
})();

First we have the Grouping operator ().

This concept is super simple. Let's see an example using the grouping operator to override the operator precedence.

const a = 1;
const b = 2;
const c = 3;

// default precedence: execute the `b * c` calculation first and then sum to `a`
a + b * c; // 7

// evaluated by default like this
a + b * c; // 7

// using the grouping operator to overriding precedence
// addition occurs before multiplication
(a + b) * c; // 9

For IIFEs, we also group an expression. In this case, the expression is an anonymous function. This part is crucial, first because it allows the JavaScript engine to execute the function when using the () to call the anonymous function. And second because everything created inside the grouping operator (lexical scope) will only be accessible from within not outside the grouping operator, which means, we don't create anything in the global scope.

Before, we didn't have a proper approach for modules in JavaScript. We used IIFEs as a module pattern.

In this mini project, I want to show an example of using IIFEs as a module pattern in JavaScript.

In the JavaScript as Scripts mini project, we saw that executing scripts would create functions, variables, and so on in the global scope. With IIFEs, we avoid polluting the global scope.

Before, the users was accessible in the global scope. The browser loads and executes the load-users.js script and attaches the users variable to the global scope.

const users = ['TK', 'Kazumi', 'Kaio'];

With the IIFE, we can make the users private and make a getter function accessible in the global scope.

const User = (function () {
  const users = ['TK', 'Kazumi', 'Kaio'];

  function getUsers() {
    return users;
  }

  return { getUsers };
})();

Here we just use an IIFE to make the users variable private and expose only a getter function.

So now, we won't be able to access the users variable outside the IIFE anymore. To use the data, we need to call the getUsers function.

Users.getUsers();

The Users is accessible in the global scope together with the public function getUsers but now the users data is private.

And we also use IIFE for the list-users.js script. We don't need to expose functions and variables it creates. It just need to execute the script, meaning: list users and add event listeners to remove users from the list.

(function () {
  const ul = document.getElementById('list');

  User.getUsers().forEach((user, index) => {
    const listItem = `li-${index}`;
    const deleteButton = document.createElement('button');

    const li = document.createElement('li');

    li.innerHTML = user;
    li.setAttribute('id', listItem);

    deleteButton.innerHTML = 'X';
    deleteButton.addEventListener('click', () => {
      li.remove();
    });

    li.appendChild(deleteButton);

    ul.appendChild(li);
  });
})();

Before, the listUsers was exposed in the global scope but now it is an anonymous function and the script is only executed, no variables or functions attached to the global scope.

In the Scope & Closure post, we saw how powerful closures are. Closures together with IIFEs are even more powerful and a good combination to build the module pattern, making it easier to not pollute the global scope and separate scripts in "modules", and keep the desired behavior of scripts.

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