Loops: Repeating Work

Computers shine at repetition. A loop runs the same block again with small changes each pass.

The for loop

for (let i = 1; i <= 5; i++) {
  console.log("Visitor number " + i);
}

Three parts, separated by semicolons:

1. Start: let i = 1 — runs once, before anything else
2. Condition: i <= 5 — checked before every pass; loop stops when false
3. Step: i++ — runs after every pass (adds 1)

So this logs 1, 2, 3, 4, 5 — five passes.

Counting from 0 — the classic trap

Arrays (next lesson) start at index 0, so most loops look like:

for (let i = 0; i < 5; i++) {
  // 0,1,2,3,4 — exactly 5 passes
  console.log(i);
}

Note the condition: i < 5, not i <= 5. Starting at 0 with < gives exactly 5 passes; mixing up the two is the most common beginner loop bug. Check yours against this rule every time.

while — loop until something changes

let cups = 3;

while (cups > 0) {
  console.log("Cups left: " + cups);
  cups = cups - 1; // something must change, or the loop never ends!
}

while is best when you do not know how many passes you need. If the condition never becomes false you get an infinite loop — the tab freezes.

Looping with a purpose: accumulating

let total = 0;
for (let n = 1; n <= 10; n++) {
  total = total + n; // add each number into the running total
}
console.log(total); // 55

The pattern — declare an accumulator before the loop, update it inside — powers sums, counting, building strings, and most data processing you will do this module.

What you learned

• for: start; condition; step
• Count from 0 with i < n; from 1 through n with i <= n
• while repeats until its condition goes false — always change something inside
• Accumulator pattern: declare before, update inside

Next: bundling code into reusable functions.