Loops: repeating work

Why loops?

Imagine printing a multiplication table, adding 50 prices or checking 1,000 roll numbers. Writing each line by hand is slow and error-prone. A loop does the repeating for you.

The for loop

for (let i = 1; i <= 5; i++) {
  console.log(`7 x ${i} = ${7 * i}`);
}
// 7 x 1 = 7 ... 7 x 5 = 35

1. let i = 1 runs once at the start.
2. i <= 5 is checked before every round; when it is false, the loop stops.
3. The body runs.
4. i++ runs after each round, then back to step 2.

for...of: every item

const prices = [120, 45, 60];
let total = 0;
for (const price of prices) {
  total += price; // same as total = total + price
}
console.log(total); // 225

for (const letter of 'UPI') {
  console.log(letter); // U, P, I
}

The while loop

while repeats as long as its condition is true. Use it when you do not know in advance how many rounds you need, for example "keep doubling the savings until they cross 1 lakh".

let savings = 1000;
let months = 0;
while (savings < 2000) {
  savings = savings * 1.1;
  months++;
}
console.log(months); // 8

break and continue

break stops the loop at once (for example, when you have found what you were looking for). continue skips the rest of this round and moves to the next one (for example, to skip empty entries).