Immediately invoked function expression: Difference between revisions

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{{Short description|Javascript design pattern}}

An '''immediately invoked function expression''' (or '''IIFE''', pronounced "iffy", [[International Phonetic Alphabet|IPA]] /ˈɪf.i/) is a [[Programming idiom|programming language idiom]] which produces a [[scope (computer science)|lexical scope]] using [[function scoping]]. It was popular in [[JavaScript]]<ref name="Alman">{{cite web|url=http://benalman.com/news/2010/11/immediately-invoked-function-expression/|title=Immediately Invoked Function Expressions|last=Alman|first=Ben|date=15 November 2010|website=|url-status=live|archive-url=https://web.archive.org/web/20171201033208/http://benalman.com/news/2010/11/immediately-invoked-function-expression/|archive-date=1 December 2017|accessdate=18 January 2019}}</ref> as a method toof supportsupporting [[modular programming]] before the introduction of more standardized solutions such as [[CommonJS]] and [[ECMAScript#6th Edition – ECMAScript 2015|ES modules]].<ref>{{cite web |last1=McGinnis |first1=Tyler |title=JavaScript Modules: From IIFEs to CommonJS to ES6 Modules |url=https://ui.dev/javascript-modules-iifes-commonjs-esmodules/ |website=ui.dev |access-date=18 August 2021 |language=en |date=15 January 2019}}</ref>

Immediately invoked function expressions can be used to avoid [[JavaScript syntax#Scoping and hoisting|variable hoisting]] from within blocks, protectprotecting against polluting the [[Global variable|global environment]] and simultaneously allowallowing public access to methods while retaining privacy for variables defined within the function. In other words, it wraps functions and variables, keeping them out of the global scope and giving them a local scope.

== Usage ==

Immediately invoked function expressions may be written in a number of different ways.<ref name=Enlighten>{{cite book |last=Lindley |first=Cody |title=JavaScript Enlightenment |year=2013 |publisher=O'Reilly |isbn=978-1-4493-4288-3 |page=61}}</ref> A [[Coding conventions|common convention]] is to enclose the function expression{{spnd}}and optionally its invocation operator{{spnd}}with the grouping operator,<ref>{{cite web |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Grouping |title=Grouping operator |date=2 October 2023 |publisher=Mozilla Developer Network}}</ref> in parentheses, to tell the parser explicitly to expect an expression. Otherwise, in most situations, when the parser encounters the <code>function</code> keyword, it treats it as a function declaration (statement), and not as a function expression.<ref>{{cite book |last=Zakas |first=Nicholas |title=Maintainable JavaScript |year=2012 |publisher=O'Reilly |isbn=978-1-4493-2768-2 |page=44}}</ref><ref>{{cite web |url=http://exploringjs.com/es6/ch_arrow-functions.html#iiaf |title=ExploringJS |author=Axel Rauschmayer}}</ref>

<syntaxhighlight lang="JavaScript">

(function () { /* ... */ })();

(function () { /* ... */ }());

(() => { /* ... */ })(); // With ES6 arrow functions (though parentheses only allowed on outside)

</syntaxhighlight>

There are other ways to enforce a function expression:

<syntaxhighlight lang="JavaScript">

!function () { /* ... */ }();

~function () { /* ... */ }();

-function () { /* ... */ }();

+function () { /* ... */ }();

void function () { /* ... */ }();

</syntaxhighlight>

In contexts where an expression is expected, wrapping in parentheses is not necessary:

<syntaxhighlight lang="JavaScriptjavascript">

varlet f = function () { /* ... */ }();

true && function () { /* ... */ }();

0, function () { /* ... */ }();

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Passing variables into the scope is done as follows:

<syntaxhighlight lang="JavaScriptjavascript">

(function(a, b) { /* ... */ })("hello", "world");

!(function (a="hello", b="world") { /* ... */ })(); //also works

</syntaxhighlight>

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==Examples==

The key to understanding design patterns such as IIFE is to realize that prior to ES6, JavaScript only featured [[Scope (computer science)#Function scope|function scope]] (thus lacking [[Scope (computer science)#Block scope|block scope]]), passing [[Call_by_reference|values by reference]] inside [[Closure (computer science)|closure]]s.<ref>{{cite book |last=Haverbeke |first=Marijn |title=Eloquent JavaScript |year=2011 |publisher=No Starch Press |isbn=978-1-59327-282-1 |pages=29–30}}</ref> This is no longer the case, as the ES6 version of JavaScript implements block scoping using the new <code>let</code> and <code>const</code> keywords.<ref>ECMAScript{{cite 6:web New|last1=Orendorff Features|first1=Jason |title=ES6 In Depth: Overviewlet &and Comparison,const [http|url=https://es6-featureshacks.mozilla.org/#BlockScopedVariables Block2015/07/es6-Scopedin-depth-let-and-const/ Variables|website=Mozilla Hacks – the Web developer blog |publisher=[[Mozilla]] |access-date=16 October 2024 |date=31 Jul 2015}}</ref><syntaxhighlight lang="javascript">

// Before ES6: Creating a scope using an IIFE

var foo = 1;

var bar = 2;

(function(){

var foo = 3; // shadows the outer `foo`

bar = 4; // overwrites the outer `bar`

})();

console.log(foo, bar); // 1 4

// Since ES6: Creating a scope using curly brackets in combination with let and const

const foo = 1;

let bar = 2;

{

const foo = 3; // shadows the outer `foo`

bar = 4; // overwrites the outer `bar`

}

console.log(foo, bar); // 1 4

</syntaxhighlight>

=== Evaluation context ===

A lack of block scope means that variables defined inside (for example) a [[for loop]] will have their definition "hoisted" to the top of the enclosing function. Evaluating a function that depends on variables modified by the outer function (including by iteration) can be difficult. We can see this without a loop if we update a value between defining and invoking the function.<ref>{{cite web |last=Alman |first=Ben |title=simple-iife-example.js |url=https://gist.github.com/cowboy/4710214 |work=Github |accessdate=5 February 2013}}</ref>

<syntaxhighlight lang="JavaScriptjavascript">

varlet v, getValue;

v = 1;

getValue = function () { return v; };

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While the result may seem obvious when updating <code>v</code> manually, it can produce unintended results when <code>getValue()</code> is defined inside a loop.

Hereafter the function passes <code>v</code> as an argument and is invoked immediately, preserving the inner function's execution context.<ref name=JQ>{{cite book |lastlast1=Otero |firstfirst1=Cesar |last2=Larsen |first2=Rob |title=Professional jQuery |year=2012 |publisher=John Wiley & Sons |isbn=978-1-118-22211-9 |page=31}}</ref>

<syntaxhighlight lang="JavaScriptjavascript">

varlet v, getValue;

v = 1;

getValue = (function (x) {

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This is equivalent to the following code:

<syntaxhighlight lang="JavaScriptjavascript">

varlet v, getValue;

v = 1;

function f(x) {

Line 83 ⟶ 95:

getValue(); // 1

</syntaxhighlight>

David Herman's ''Effective JavaScript'' contains an example illustrating the problems of evaluation context inside loops.<ref>{{cite book |last=Herman |first=David |title=Effective Javascript |year=2012 |publisher=Addison-Wesley |isbn=978-0-321-81218-6 |pages=44–45}}</ref> While Herman's example is deliberately convoluted, it arises directly from the same lack of block scope.<ref>{{cite book |last=Zakas |first=Nicholas C. |title=Professional JavaScript for Web Developers |chapter=Mimicking Block Scope |year=2011 |publisher=John Wiley & Sons |isbn=978-1-118-23309-2}}</ref>

=== Establishing private variables and accessors ===

IIFEs are also useful for establishing private methods for accessible functions while still exposing some properties for later use.<ref>{{cite book |last=Rettig |first=Pascal |title=Professional HTML5 Mobile Game Development |year=2012 |publisher=John Wiley & Sons |isbn=978-1-118-30133-3 |page=145}}</ref> The following example comes from Alman's post on IIFEs.<ref name=Alman/>

<syntaxhighlight lang="JavaScriptjavascript">

// "counter" is a function that returns an object with properties, which in this case are functions.

varlet counter = (function () {

varlet i = 0;

return {