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Functions are reusable blocks of code that take inputs (parameters) and return an output. Three reasons to write a function: reuse (call it many times), abstraction (give a meaningful name to a multi-step operation), and isolation (variables inside don't leak out).

Function declaration

Four pieces:

1. The function keyword.
2. The function name (camelCase, descriptive).
3. Parameters in parens — local names that receive values when called.
4. A { ... } body — usually with a return statement.

Function expressions — assign to a variable

Functions are FIRST-CLASS values in JavaScript: you can store them in variables, pass them as arguments, return them. Both forms work; the difference is hoisting:

• Function declarations are hoisted — usable BEFORE the declaration line.
• Function expressions are not — using them before assignment raises ReferenceError.

Parameters and arguments

Missing arguments default to undefined. Extra arguments are silently ignored. Use default parameters to be explicit:

Rest parameters — variable-length args

...nums collects all remaining arguments into an array.

Spread at the call site

Same ... syntax — but at a call, it UNPACKS an array into separate arguments.

Implicit return of undefined

Without an explicit return, the function returns undefined. Don't confuse console.log (output) with return (the function's value).

Common mistakes

• Forgetting return — function silently returns undefined.
• Calling without parens: console.log(greet) logs the function itself, not its result.
• Modifying a passed object thinking it's a copy — objects are passed by reference. Mutating one inside changes the caller's.
• Missing default params — function f(x) called with f() makes x = undefined.