Loops

for Loop

The classic for loop repeats code a specific number of times.

for (let i = 0; i < 5; i++) {
  console.log("Count: " + i);
}

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while Loop

A while loop runs as long as its condition remains true.

let count = 0;
while (count < 3) {
  console.log("Tick " + count);
  count++;
}

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do...while Loop

A do...while loop always executes its body at least once before checking the condition.

let num = 10;
do {
  console.log("Number: " + num);
  num++;
} while (num < 10);
// Runs once even though condition is false

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for...in Loop

The for...in loop iterates over enumerable property keys of an object.

const car = { make: "Toyota", model: "Camry", year: 2022 };
for (const key in car) {
  console.log(key + ": " + car[key]);
}

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for...of Loop

The for...of loop iterates over iterable values (arrays, strings, etc.).

const colors = ["red", "green", "blue"];
for (const color of colors) {
  console.log(color);
}

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Looping Backwards

You can decrement the loop counter to traverse an array in reverse.

const items = ["A", "B", "C"];
for (let i = items.length - 1; i >= 0; i--) {
  console.log(items[i]);
}

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Nested Loops

Loops inside loops create combinations or grids.

for (let row = 1; row <= 3; row++) {
  let line = "";
  for (let col = 1; col <= row; col++) {
    line += "* ";
  }
  console.log(line);
}

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Array Methods as Loops

Methods like forEach, map, and filter are modern alternatives to manual loops.

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(n => n * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

const evens = numbers.filter(n => n % 2 === 0);
console.log(evens); // [2, 4]

numbers.forEach(n => console.log(n * 3));

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🧪 Quick Quiz

What does for...of iterate over?

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