Control flow · Lesson 12 of 45

Loops: for and while

Repeat something without writing it again and again.

9 minutes4 games

A for loop has 3 parts in its brackets, separated by ;:

start: let i = 1 (runs once).
condition: i <= 3 (keep going while true).
step: i++ (after every round).

Step by step1 / 11

▶let total = 0
2for (let i = 1; i <= 3; i++) {
3 total += i
4 console.log("i =", i, "total =", total)
5}

Variables

total

Output

…

Press “Play” and watch i and total change.

while and do…while

while repeats while its condition is true; it suits cases where you don't know the number of rounds in advance. do…while is the same but always runs at least once before checking.

JavaScript

let coins = 3
while (coins > 0) {
  console.log("Spend a coin, left:", coins - 1)
  coins--
}

let tries = 0
do {
  tries++
} while (tries < 1)
console.log("tries:", tries)

break and continue

JavaScript

for (let i = 1; i <= 10; i++) {
  if (i % 2 === 0) continue  // skip even numbers
  if (i > 7) break           // stop completely
  console.log(i)
}

Complete the code

Complete the loop to print 0 to 4.

for (let i = ; i < ; i++) {
  console.log(i)
}

Spot the bug

This loop never ends. Where's the problem?

What's the output?

for (let i = 0; i < 6; i++) {
  if (i === 2) continue
  if (i === 4) break
  console.log(i)
}

What will this code print?

Exercise

Print the sum of 1 to 10 (the result is 55).

Expected output: 55

JavaScript

let sum = 0

Learn more from

Played 0 of 4 games

JavaScript

Control flow · Lesson 12 of 45

Loops: for and while

Repeat something without writing it again and again.

9 minutes4 games

A for loop has 3 parts in its brackets, separated by ;:

start: let i = 1 (runs once).
condition: i <= 3 (keep going while true).
step: i++ (after every round).

Step by step1 / 11

▶let total = 0
2for (let i = 1; i <= 3; i++) {
3 total += i
4 console.log("i =", i, "total =", total)
5}

Variables

total

Output

…

Press “Play” and watch i and total change.

while and do…while

while repeats while its condition is true; it suits cases where you don't know the number of rounds in advance. do…while is the same but always runs at least once before checking.

JavaScript

let coins = 3
while (coins > 0) {
  console.log("Spend a coin, left:", coins - 1)
  coins--
}

let tries = 0
do {
  tries++
} while (tries < 1)
console.log("tries:", tries)

break and continue

JavaScript

for (let i = 1; i <= 10; i++) {
  if (i % 2 === 0) continue  // skip even numbers
  if (i > 7) break           // stop completely
  console.log(i)
}

Complete the code

Complete the loop to print 0 to 4.

for (let i = ; i < ; i++) {
  console.log(i)
}

Spot the bug

This loop never ends. Where's the problem?

What's the output?

for (let i = 0; i < 6; i++) {
  if (i === 2) continue
  if (i === 4) break
  console.log(i)
}

What will this code print?

Exercise

Print the sum of 1 to 10 (the result is 55).

Expected output: 55

JavaScript

let sum = 0

Learn more from

Played 0 of 4 games