Functions: Reusable Code

You have already used functions: console.log and name.toUpperCase() are functions someone else wrote. Now you write your own.

Declaring a function

function greet(name) {
  return "Hello, " + name + "!";
}

greet("Ada"); // "Hello, Ada!"
greet("Grace"); // "Hello, Grace!" — same work, different input

Anatomy:

• function — the keyword that starts a declaration
• greet — the name (verb-like: functions do things)
• (name) — parameters: named inputs the function expects
• return — hands a value back to whoever called the function

Parameters vs arguments

Parameters are the names in the definition; arguments are the actual values passed in a call:

function add(a, b) {
  // a, b are parameters
  return a + b;
}

add(2, 3); // 2 and 3 are arguments → 5

return ends the function

The moment return runs, the function is done — code below it is skipped. A function with no return gives back undefined:

function logTwice(msg) {
  console.log(msg);
  console.log(msg);
} // returns undefined — does work, hands nothing back

function double(n) {
  return n * 2;
} // hands 4 back when called with 2

Both styles are legitimate: do something (log, save, update the page) versus compute something (return it). Graders in this course usually call your function, so returning is how you hand results over.

Why functions matter

Functions let you write logic once and trust it everywhere. Fix a bug inside greet and every call site is fixed. They are also how bigger programs stay readable: a well-named function is a one-line summary of what it does.

What you learned

• Declare with function name(parameters) { ... }
• return hands a value back and ends the function
• No return → undefined
• Write logic once, call it many times

Next: a shorter syntax for functions, and how variable visibility works.