{{Short description|Javascript design pattern}}
An '''immediately invoked function expression''' (or '''IIFE''', pronounced "iffy", IPA /ˈɪf.i/) is a programming language idiom which produces a 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 of supporting modular programming before the introduction of more standardized solutions such as CommonJS and 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 variable hoisting from within blocks, protecting against polluting the global environment and simultaneously allowing 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 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>
In contexts where an expression is expected, wrapping in parentheses is not necessary: <syntaxhighlight lang="javascript"> let f = function () { /* ... */ }(); true && function () { /* ... */ }(); 0, function () { /* ... */ }(); </syntaxhighlight>
Passing variables into the scope is done as follows: <syntaxhighlight lang="javascript"> (function(a, b) { /* ... */ })("hello", "world"); (function(a="hello", b="world") { /* ... */ })(); // also works </syntaxhighlight>
An initial parenthesis is one case where the automatic semicolon insertion (ASI) in JavaScript can cause problems; the expression is instead interpreted as a call to the last term on the preceding line. In some styles that omit optional semicolons, the semicolon is placed ''in front'' of the parenthesis, and is known as a defensive semicolon.<ref name=inimino>{{cite web |url=http://inimino.org/~inimino/blog/javascript_semicolons |title=JavaScript Semicolon Insertion: Everything you need to know |date=28 May 2010 |archive-url=https://web.archive.org/web/20171002224530/http://inimino.org/~inimino/blog/javascript_semicolons |archive-date=2 October 2017 |url-status=live}}</ref><ref>{{cite web |url=https://mislav.net/2010/05/semicolons/ |title=Semicolons in JavaScript are optional |first=Mislav |last=Marohnić |date=7 May 2010 |archive-url=https://web.archive.org/web/20170808231150/https://mislav.net/2010/05/semicolons/ |archive-date=8 August 2017 |url-status=live}}</ref> For example: <syntaxhighlight lang="JavaScript"> a = b + c ;(function () { // code })(); </syntaxhighlight> ...to avoid being parsed as <code>c()</code>.
==Examples== The key to understanding design patterns such as IIFE is to realize that prior to ES6, JavaScript only featured function scope (thus lacking block scope), passing values by reference inside closures.<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>{{cite web |last1=Orendorff |first1=Jason |title=ES6 In Depth: let and const |url=https://hacks.mozilla.org/2015/07/es6-in-depth-let-and-const/ |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="javascript"> let v, getValue; v = 1; getValue = function () { return v; }; v = 2;
getValue(); // 2 </syntaxhighlight>
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 |last1=Otero |first1=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="javascript"> let v, getValue; v = 1; getValue = (function (x) { return function () { return x; }; })(v); v = 2;
getValue(); // 1 </syntaxhighlight>
This is equivalent to the following code: <syntaxhighlight lang="javascript"> let v, getValue; v = 1; function f(x) { return function () { return x; }; }; getValue = f(v); v = 2;
getValue(); // 1 </syntaxhighlight>
=== 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="javascript"> // "counter" is a function that returns an object with properties, which in this case are functions. let counter = (function () { let i = 0;
return { get: function () { return i; }, set: function (val) { i = val; }, increment: function () { return ++i; } }; })();
// These calls access the function properties returned by "counter". counter.get(); // 0 counter.set(3); counter.increment(); // 4 counter.increment(); // 5 </syntaxhighlight>
If we attempt to access <code>counter.i</code> from the global environment, it will be undefined, as it is enclosed within the invoked function and is not a property of <code>counter</code>. Likewise, if we attempt to access <code>i</code>, it will result in an error, as we have not declared <code>i</code> in the global environment.
== Terminology == Originally known as a "self-executing anonymous function",<ref>{{cite book |last=Resig |first=John |title=Pro JavaScript Techniques |year=2006 |publisher=Apress |isbn=978-1-4302-0283-7 |page=29}}</ref> Ben Alman later introduced the current term IIFE as a more semantically accurate name for the idiom, shortly after its discussion arose on comp.lang.javascript.<ref name=Alman/><ref name=Osmani>{{cite book |last=Osmani |first=Addy |title=Learning JavaScript Design Patterns |year=2012 |publisher=O'Reilly |isbn=978-1-4493-3487-1 |page=206}}</ref><ref>{{cite news |last=Baagoe |first=Johannes |title=Closing parenthesis in function's definition followed by its call |url=https://groups.google.com/forum/#!topic/comp.lang.javascript/tjVn1NjGDN8%5B1-25%5D |accessdate=19 April 2010}}</ref>
Notably, immediately invoked functions need not be anonymous inherently, and ECMAScript{{nbsp}}5's strict mode forbids <code>arguments.callee</code>,<ref>{{cite web |title=Strict mode |url=https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Functions_and_function_scope/Strict_mode#Making_eval_and_arguments_simpler |work=Mozilla JavaScript Reference |publisher=Mozilla Developer Network |accessdate=4 February 2013}}</ref> rendering the original term a misnomer.
== See also == *Evaluation strategy
== References == {{reflist|30em}}
== External links == * {{cite web |title=Functions and function scope |url=https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Functions_and_function_scope |work=Mozilla JavaScript Reference |publisher=Mozilla Developer Network |accessdate=4 February 2013}} Category:JavaScript Category:Programming language concepts