What is an IIFE in JavaScript, and why was it historically common?

Direct answer

An IIFE is a function expression invoked immediately after it is created; it was widely used to create private function scope and avoid global variables before block scope and ES modules became standard.

A typical form is (function(){ const privateValue = 1; })(); or the arrow equivalent (() => { /* setup */ })();. The surrounding parentheses force the parser to treat the function as an expression, and the final parentheses invoke it.

Older browser code used IIFEs to isolate variables because var is function-scoped and script files otherwise shared globals.
The module pattern used IIFEs plus closures to expose a small public API while keeping implementation details private.
Today, let, const, block scope, and ES modules remove many historical reasons for IIFEs.
Async IIFEs can still be useful when an immediate async execution wrapper is convenient in a context that cannot use top-level await.