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Loops (for & while)

Learn how loops repeat code automatically — for loops for counted repeats and while loops for conditional ones.

What you'll learn

  • Why loops exist and what they replace
  • How a for loop counts through a range
  • How a while loop repeats until a condition fails
  • How to avoid infinite loops

Concept

Loops (for & while)

Loops repeat a block of code multiple times. Without loops, you would have to copy and paste the same lines for every repetition — imagine printing "Hello" 100 times by hand. Loops let you write the instruction once and say "do this N times."

The for Loop

A for loop is used when you know how many times to repeat. It has three parts: initialization, condition, and update.

for (let i = 0; i < 5; i++) {
  console.log("Iteration", i);
}
// Prints: Iteration 0, 1, 2, 3, 4
  • let i = 0 — runs once, before the loop starts.
  • i < 5 — checked before each iteration; the loop continues while true.
  • i++ — runs after each iteration, incrementing the counter.

The while Loop

A while loop is used when you do not know how many repetitions you need — you repeat until a condition becomes false.

let dice = 0;
while (dice !== 6) {
  dice = Math.floor(Math.random() * 6) + 1;
  console.log("Rolled:", dice);
}

Beware Infinite Loops

An infinite loop never stops because its condition never becomes false. This freezes the program. Always make sure the loop's progress eventually breaks the condition — for example, that i increases toward the limit, or that dice can actually reach 6.

Use break to exit a loop early and continue to skip to the next iteration when you need finer control.

Example

              // for loop: count down from 5
for (let i = 5; i > 0; i--) {
  console.log(i);
}
console.log("Liftoff!");

// while loop: double a number until it exceeds 100
let value = 1;
let steps = 0;
while (value <= 100) {
  value *= 2;
  steps++;
}
console.log(`Reached ${value} in ${steps} steps`);  // Reached 128 in 7 steps

The for loop counts down because i-- decreases i each step. The while loop keeps doubling until the condition (value <= 100) is false.

Try it

  • Crontab Generator

    Cron expressions are essentially loops in time — 'run every N minutes'. See how repeating schedules work.

Common mistakes

The mistake

Writing a while loop whose condition can never become false.

The fix

Ensure the loop body changes a variable used in the condition. Mentally trace: does progress happen every iteration?

The mistake

Off-by-one errors: using < instead of <=.

The fix

Decide whether the endpoint is inclusive. for (let i = 0; i <= n; i++) runs n+1 times; i < n runs n times. Test the boundary.

Related Resources

Related Snippets

Practice Loops (for & while)

2 exercises

Practice Loops (for & while)

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