Iifes

An immediately invoked function expression, or IIFE, is a JavaScript function expression that is executed as soon as it is created. IIFEs create a new execution context and scope, which helps prevent variables from leaking into the global namespace and supports encapsulation through closures.

Common syntax involves wrapping a function expression in parentheses and then invoking it with another pair of parentheses. Examples include (function(){ /* code */ })(); and (function foo(){ /* code */ })();. An alternative form is (function(){ /* code */ }()) with the invocation inside the outer parentheses. Arrow functions can also be used as IIFEs: (() => { /* code */ })();.

IIFEs are frequently used to implement the module pattern, initialize code, or set up private variables and functions. They are useful for isolating setup code, creating private state, and exporting a public API via a returned object. By executing immediately, an IIFE can run initialization logic without leaving temporary variables in scope.

In modern JavaScript, the need for IIFEs to avoid global scope has diminished due to the introduction of modules, which provide their own scope. Nevertheless, IIFEs remain common in older codebases and in situations that require immediate execution of initialization code or encapsulation of private state. When used, they should minimize global side effects and can accept parameters to alias global objects (for example, (function (win){ /* code */ })(window)).

Examples illustrate a simple counter module created by an IIFE:

var counter = (function(){ var n = 0; return { next: function(){ n++; return n; } }; })();