Function Declarations vs Expressions

When you need to reuse a function multiple times
When you need to create a function that's accessible throughout your codebase
When you need to create a private scope to avoid polluting the global namespace
An IIFE (Immediately Invoked Function Expression) is most appropriate when you need to create a private scope to avoid polluting the global namespace. The IIFE pattern creates a function expression that executes immediately: `(function() { /* code */ })();`. This creates a new scope that's isolated from the surrounding code, allowing you to declare variables and functions that won't conflict with variables in other scopes, even if they have the same name. This is particularly useful for encapsulating initialization code, creating modules with private state, avoiding global namespace pollution, and preventing variables from persisting in memory when they're no longer needed. IIFEs were especially important before ES6 modules and block-scoped variables (let/const), as they provided one of the few ways to create private scopes in JavaScript. They're still valuable for one-time execution code that needs its own scope.
When you need the best possible performance for function calls