Loops: for, while, do…while, for…of and for…in

Why loops?

Computers are good at doing the same thing many times without getting tired. A loop runs a block of code again and again. Each run is called an iteration. Without loops you would have to copy the same line a thousand times for a thousand orders.

The for loop: counting

A for loop has three parts inside the brackets, separated by semicolons: a start (let i = 0), a condition checked before every round (i < 5), and an update run after every round (i++).

for (let i = 1; i <= 5; i++) {
  console.log('Table of 7:', 7 * i);
}

while and do…while

while repeats as long as its condition is true. Use it when you do not know in advance how many rounds you need. do…while runs the body once BEFORE checking, so it always runs at least once.

let balance = 1000;
let months = 0;
while (balance > 0) {
  balance -= 300;
  months++;
}
console.log(months); // 4

let tries = 0;
do {
  tries++;
} while (tries < 0);
console.log(tries);  // 1 (the body ran once even though the condition was false)

for…of: the items of an iterable

for…of gives you each VALUE of an array, a string, a Map or a Set, one by one. It is the clearest loop when you do not need the index.

const cart = ['rice', 'dal', 'ghee'];
for (const item of cart) {
  console.log(item);
}
for (const ch of 'UPI') console.log(ch); // U, P, I

for…in: the keys of an object

for…in gives you each property NAME (key) of an object. Do not use it on arrays: the keys come out as strings ('0', '1') and it can also visit inherited or extra properties. For arrays use for…of or a normal for.

const stock = { rice: 40, dal: 25 };
for (const key in stock) {
  console.log(key, stock[key]); // rice 40, then dal 25
}

break and continue

for (const n of [3, 8, -1, 5]) {
  if (n < 0) break;      // stop the whole loop
  if (n % 2 === 0) continue; // skip even numbers
  console.log(n);
}

Labels let break leave an outer loop from inside an inner one (outer: for (...) { for (...) { break outer; } }). They are rarely needed; a function with return is usually clearer.