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An IIFE (Immediately-Invoked Function Expression) is a function that runs as soon as it's defined:

The wrapping parens are needed to make the parser treat it as an expression. Without them, JavaScript thinks function() {} is a declaration and complains.

IIFEs were the classic way to create private scope before ES6 modules. The module pattern:

In modern code, ES modules (import/export) replaced this — but you'll still see IIFEs in legacy code, bundles, and some library code.