Loops

Introduction

Imagine writing "I will listen carefully" 100 times. You would get tired quickly. A computer would not. You tell it once to repeat. That is what a loop does.

The simple explanation

A loop repeats a block of code.

The for loop

A for loop has three parts in the brackets, separated by semicolons:

1. Start: let i = 1 creates a counter.
2. Condition: i <= 5 keeps looping while this is true.
3. Step: i++ adds 1 to the counter after each round.

The code in the curly braces runs once for each round. i++ is shorthand for i = i + 1.

Adding things up

Loops are great for building a result step by step.

This adds 1 + 2 + 3 + 4 + 5 and prints 15.

The while loop

A while loop keeps going as long as its condition is true. Use it when you do not know in advance how many rounds you need.

Be careful: if the condition never becomes false, the loop never ends. Try changing n * 2 to n and running it. This site will stop the program after a few seconds.

for...of

To go through the items of an array, for...of is the cleanest way.

break and continue

• break stops the loop completely.
• continue skips the rest of this round and moves to the next.

It prints 1, 2, 4 and 5: it skips 3, and stops before printing 6.

Try it yourself

Print the numbers 1 to 10, but only the even ones. Hint: use % and an if inside the loop.

Summary

• A loop repeats code. for is great when you know the number of rounds.
• while repeats as long as a condition is true. Make sure it can end.
• for...of walks through an array.
• break stops a loop and continue skips a round.