JavaScript — Loops

Why loops exist

Print numbers 1–5 without a loop:

console.log(1); console.log(2); console.log(3); console.log(4); console.log(5);

Now imagine 1–5000. Loops repeat a block as many times as needed:

for — the counting classic

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

Three clauses, read once slowly:

for ( let i = 1 ;  i <= 5 ;  i++ )
      ──┬──────   ───┬───   ─┬─
     start      keep going?  step after each pass

1. let i = 1 — runs once before everything
2. i <= 5 — checked before every iteration; false ends the loop
3. i++ — runs after each iteration

What you see

1
2
3
4
5

Counting down, skipping, stepping by 10 — all just clause edits:

for (let i = 10; i >= 0; i -= 2) { … }   // 10,8,6,4,2,0

for...of — visiting each item

const fruits = ["apple", "banana", "mango"];

for (const fruit of fruits) {
    console.log(fruit);
}

No counters, no off-by-one risk. The default choice for arrays/strings:

for (const ch of "abc") console.log(ch);   // a b c

while — repeat until condition dies

let fuel = 3;

while (fuel > 0) {
    console.log("Vroom");
    fuel--;
}

Use when you don't know the count upfront — waiting for user input, draining a queue.

The infinite-loop hazard: forgetting to change the condition freezes the tab:

while (true) { }        // browser locks. Don't.

Always ensure something inside moves toward falseness.

do...while — check AFTER

let answer;
do {
    answer = prompt("Type 'go'");     // body runs at least ONCE
} while (answer !== "go");

Only difference from while: condition tested after the first pass.

Breaking early & skipping

for (const n of [1, 7, 0, 9]) {
    if (n === 0) continue;    // skip just this iteration
    if (n > 8) break;         // abandon whole loop
    console.log(n);
}
// prints 1, 7 then stops before 9

Choosing

Situation	Loop
Known count / index math	for (let i…)
Every item of array/string	for...of
Unknown repetitions	while
Must run at least once	do...while

(Modern code often skips explicit loops entirely via map/filter/reduce — its own lesson.)

Common mistakes: < vs <= off-by-one; mutating the array while iterating it; infinite loops from forgotten increments.

Mini Practice

1. Multiples of 7 up to 70 with a for
2. Sum all numbers in an array with for...of
3. Countdown 10→1 then log "Liftoff!"
4. Number-guessing game loop with do...while + prompt
5. Demo continue and break inside one loop; predict output first

Next: break → (next new topic: maps)