IIFE

An immediately invoked function expression (IIFE) is a JavaScript pattern that defines a function as part of a function expression and then executes it right away. By creating a new function scope, an IIFE helps prevent variables from leaking into the global namespace and can encapsulate private state.

Common syntax patterns include (function(){ /* code */ })(); and (function(){ /* code */ }());. These constructs define a function expression and immediately invoke it. Named IIFEs, such as (function myModule(){ /* code */ })();, are useful for recursion and easier debugging. Other variants use operators to force an expression, for example void function(){ /* code */ }(); or +function(){ /* code */ }();.

Variants in modern JavaScript include arrow-function IIFEs, e.g., (() => { /* code */ })(); and async IIFE patterns like (async () => { /* code */ })();. These retain the same immediate execution behavior while adapting to newer syntax.

Use cases include implementing the module pattern, initializing code once, and creating a private scope for variables and helper functions. An IIFE can return an object to expose a public API while keeping internal details private, thereby reducing global pollution and enabling closures.

History and relation to modules: IIFEs were a primary means of creating modular code in environments without native module support. With the advent of ES6 modules, the need for IIFEs for scope management diminished, but they remain in legacy code and certain initialization scenarios. They also enable strict mode enforcement within a contained scope by placing "use strict" inside the function.