What IIFEs Actually Do
An Immediately Invoked Function Expression runs the moment it's defined: (function() { /* code */ })() or the arrow version (() => {})(). It creates private lexical scope without polluting the global namespace. Before ES6 (circa 2015), this was the standard pattern for encapsulation in JavaScript. Think jQuery plugins or any library that needed to avoid variable conflicts.
Why They Mattered Then
Pre-ES6, JavaScript had no block scope with var. IIFEs solved this by wrapping code in a function scope. Popular libraries leaned heavily on the pattern: jQuery used it for plugin isolation, module loaders built on it. Ben Alman popularized the term around 2010, and it became standard practice for avoiding global pollution in browser code.
Why They Don't Matter Now
ES6's let and const provide block scope without function overhead. ES6 modules (import/export) handle encapsulation better for anything beyond single-file scripts. Modern bundlers (Webpack, Vite) manage scope automatically. The GeeksforGeeks update from July 2025 still covers IIFEs as a teaching tool, but it's telling that no major framework or library has shipped new IIFE-centric patterns in years.
Where They Still Show Up
Legacy enterprise codebases. Initialization scripts that need to run once without globals. Async/await execution in older Node.js environments lacking top-level await. Some teams use them for quick scope isolation in React component files or TypeScript when refactoring isn't worth the effort.
The Trade-offs
IIFEs add a function call to the stack. ES6 block scope is cleaner and more performant for most cases. Memory leak patterns exist with IIFE closures in Node.js production code, particularly when variables capture large objects unintentionally. Modern tooling (heapdump, PM2 profiling) can surface these issues, but prevention is simpler: use modules.
What This Means in Practice
If you're writing new code, use ES6 modules and block scope. If you're maintaining legacy systems with IIFEs, they're not a priority for refactoring unless they're causing actual problems (memory leaks, debuggability issues). The pattern isn't harmful, it's just obsolete. Your team's time is better spent on architecture that matters to the business.
History suggests: when a language feature gets superseded by better primitives, the old pattern lingers in codebases for years but stops appearing in new projects. IIFEs crossed that line around 2017. They're not wrong, they're just done.