Immediately invoked function expression

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 as a method to support modular programming before the introduction of more standardized solutions such as CommonJS and ES modules.

Immediately invoked function expressions can be used to avoid variable hoisting from within blocks, protect against polluting the global environment and simultaneously allow public access to methods while retaining privacy for variables defined within the function.

Usage
Immediately invoked function expressions may be written in a number of different ways. A common convention is to enclose the function expression –and optionally its invocation operator –with the grouping operator, in parentheses, to tell the parser explicitly to expect an expression. Otherwise, in most situations, when the parser encounters the  keyword, it treats it as a function declaration (statement), and not as a function expression.

There are other ways to enforce a function expression:

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

Passing variables into the scope is done as follows:

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. For example: ...to avoid being parsed as.

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. This is no longer the case, as the ES6 version of JavaScript implements block scoping using the new  and   keywords.

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.

While the result may seem obvious when updating  manually, it can produce unintended results when   is defined inside a loop.

Hereafter the function passes  as an argument and is invoked immediately, preserving the inner function's execution context.

This is equivalent to the following code:

Establishing private variables and accessors
IIFEs are also useful for establishing private methods for accessible functions while still exposing some properties for later use. The following example comes from Alman's post on IIFEs.

If we attempt to access  from the global environment, it will be undefined, as it is enclosed within the invoked function and is not a property of. Likewise, if we attempt to access, it will result in an error, as we have not declared   in the global environment.

Terminology
Originally known as a "self-executing anonymous function", 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.

Notably, immediately invoked functions need not be anonymous inherently, and ECMAScript5's strict mode forbids, rendering the original term a misnomer.