Loops

JavaScript supports the standard loop constructs you have seen in other programming languages:

let counter = 0;
while (counter < 10) {
	console.log(counter);
	counter++;
}

In a do-while loop, the body of the loop is executed once and then, if the condition holds, the program returns to the top of the block.

let counter = 10;
do {
	console.log(counter);
	counter++;
} while (counter < 10);

When it comes to counter-controlled loops (when we know exactly how many iterations we need, e.g. when we go over the elements of an array), the for loop provides a more compact syntax:

for (let counter = 0; counter < 10; counter++) {
	console.log(counter);
}

You can cram in multiple variables, update expressions:

const arr = [10, 20, 30, 40, 50];

for (let i = 0, j = arr.length - 1; i < j; i++, j--) {
  const temp = arr[i]
  arr[i] = arr[j]
  arr[j] = temp
}

console.log(arr);

The while and do-while loops are best for event-controlled loops (when the number of iteration is unknown before runtime, e.g. when we read data from file and we don't know how many lines — or how many values are there).

break & continue

The break statement exits out of a loop, while the continue statement will skip to the next iteration:

const arr = [10, 20, 30, 40, 50];
const target = 40;

let index = -1;
for (let i = 0; i < arr.length; i++) {
  if (arr[i] === target) {
    index = i;
    break; // stop early!
  }
}

console.log(index);

const arr = [10, , 30, , 50];
let count = 0;
let sum = 0;

for (let i = 0; i < arr.length; i++) {
  if (arr[i] === undefined) {
    continue; // ignore missing data!
  }
  count++
  sum += arr[i]
}

const avg = count === 0 ? 0 : sum / count;
console.log(avg);