Function Declarations vs Expressions

The primary advantage of function expressions over function declarations is that function expressions can be used immediately as Immediately Invoked Function Expressions (IIFEs). Since function expressions produce a value (a function object) rather than a declaration, they can be invoked immediately by appending parentheses: `(function() { /* code */ })();`. This pattern is impossible with function declarations. IIFEs are valuable for creating private scopes to avoid polluting the global namespace, executing code that doesn't need to be reused, and creating closures with private variables. Function expressions are also more flexible in how they can be used: they can be passed as arguments to other functions, returned from functions, assigned conditionally, and used in all the ways that any other value can be used. Both expression and declaration types support closures and have similar performance characteristics.