The answer
By wrapping entire files in a function expression that ran immediately. var declarations inside the IIFE were scoped to the function, not the global object. Today, ES modules do this automatically.
Block Scope and the IIFE Pattern in JavaScript
How did IIFEs prevent global pollution in JavaScript?
More FAQs in Block Scope and the IIFE Pattern in JavaScript
An Immediately Invoked Function Expression. It creates a new function scope immediately. Before ES6, it was used to create block-like scope for data privacy, to avoid global pollution, and to fix the var loop closure bug.
let is block-scoped and creates a fresh binding per iteration. Each callback closes over its own i, fixing the var loop closure bug without needing an IIFE. The IIFE was the pre-ES6 workaround.
Rarely. let and const provide block scope, ES modules provide module scope (no global pollution), and regular functions provide data privacy. IIFEs are mainly needed in old codebases that do not support ES6 or modules.
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