An '''immediately- invoked function expression''' (or '''IIFE''', pronounced "iffy")<ref name=Alman>{{cite web |last=Alman |first=Ben |title=Immediately Invoked Function Expressions |url=http://benalman.com/news/2010/11/immediately- invoked-function-expression |date=15 November 2010 |accessdate=4 February 2013 |archive-url=https://web.archive.org/web/20171201033208/http://benalman.com/news/2010/11/immediately- invoked-function-expression/ |archive-date=1 December 2017 |dead-url=no}}</ref> is a [[JavaScript]] [[Programming idiom|programming language idiom]] which produces a [[scope (computer science)|lexical scope]] using JavaScript's [[function scoping]]. Immediately- invoked function expressions can be used to avoid [[JavaScript syntax#Scoping and hoisting|variable hoisting]] from within blocks, protect against polluting the [[Global variable|global environment]] and simultaneously allow public access to methods while retaining privacy for variables defined within the function. This concept has been referred to 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> but Ben Alman introduced the term IIFE as a more semantically accurate term 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>
== 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 |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 () { /* ... */ })();
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== Examples ==
The key to understanding design patterns such as immediately- invoked function expressions is to realize that until recently JavaScript had only [[Scope (computer science)#Function scope|function scope]] (but not [[Scope (computer science)#Block scope|block scope]]) and passes values by reference inside a [[Closure (computer science)|closure]].<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 entirely true, in the latest version of JavaScript block scoping is available and becomes evident when using the new <code>let</code> and <code>const</code> keywords.<ref>ECMAScript 6: New Features: Overview & Comparison, [http://es6-features.org/#BlockScopedVariables Block-Scoped Variables]</ref>
=== Evaluation context ===
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== Terminology ==
"Immediately- invoked function expression" as a term describes a design pattern that has also been referred to as a "self-executing anonymous function".<ref name=Alman/><ref name=Enlighten/> However, immediately- invoked functions do not need to be anonymous, 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> making the latter term less accurate.<ref name=Osmani/><ref name=JQ/>
In [[lambda calculus]], this construct was referred to as "redex", for reducible expression, see [[Reduction strategy (code optimization)|Reduction strategy]].
