JavaScript Loops

Loops repeat code multiple times, saving you from writing the same thing over and over.

for Loop

The most common loop. Has three parts: initialization, condition, and update:

for (let i = 0; i < 5; i++) {
  console.log(i);
}
// Output: 0, 1, 2, 3, 4

1. Initialization (let i = 0) — runs once at start
2. Condition (i < 5) — checked before each iteration
3. Update (i++) — runs after each iteration

while Loop

Use when you don’t know how many times to iterate:

let i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

The condition is checked before each iteration.

do...while Loop

Always runs at least once, even if condition is false:

let i = 0;
do {
  console.log(i);
  i++;
} while (i < 5);

Tip: Good for menus that must display at least once.

break and continue

// break — exit loop immediately
for (let i = 0; i < 10; i++) {
  if (i === 5) break;
  console.log(i);  // 0, 1, 2, 3, 4
}

// continue — skip to next iteration
for (let i = 0; i < 10; i++) {
  if (i % 2 !== 0) continue;
  console.log(i);  // 0, 2, 4, 6, 8
}

Nested Loops

for (let i = 1; i <= 5; i++) {
  let row = "";
  for (let j = 1; j <= 5; j++) {
    row += `${i * j}\t`;
  }
  console.log(row);
}

The inner loop completes all iterations for each outer loop iteration.

Complete Example: FizzBuzz

for (let i = 1; i <= 100; i++) {
  if (i % 3 === 0 && i % 5 === 0) {
    console.log("FizzBuzz");
  } else if (i % 3 === 0) {
    console.log("Fizz");
  } else if (i % 5 === 0) {
    console.log("Buzz");
  } else {
    console.log(i);
  }
}

Complete Example: Find Value

let numbers = [3, 7, 1, 9, 4, 2, 8];
let target = 9;
let found = false;

for (let i = 0; i < numbers.length; i++) {
  if (numbers[i] === target) {
    console.log(`Found ${target} at index ${i}`);
    found = true;
    break;
  }
}

Best Practices

1. Use for when count is known — More readable
2. Use while when condition-driven — More flexible
3. Avoid infinite loops — Always have an exit condition
4. Use break wisely — Exit early when done
5. Use continue wisely — Skip without nesting

Common Mistakes

1. Infinite loops — Missing increment or wrong condition
2. Off-by-one errors — Loop runs one too many/few times
3. Using = instead of === — Assignment in condition
4. Forgetting braces — Only first line runs in loop
5. Modifying array while iterating — Can skip elements