1.9Lesson 1.9: Conditions and loops in JavaScript

The idea

The same ideas as in 1.4 and 1.5. Two things change:

• The condition goes in brackets.
• What's inside goes between curly braces, instead of ending with EndIf or EndWhile.

if and else

const age = Number(prompt("How old are you?"));

if (age >= 18) {
  console.log("You can vote");
} else {
  console.log("You can't vote yet");
}

For more than two paths, else if:

if (n > 0) {
  console.log("Positive");
} else if (n < 0) {
  console.log("Negative");
} else {
  console.log("Zero");
}

Comparing and combining

Careful: = stores and === compares. Mixing them up is the most common mistake. You'll also see ==; always use ===, which doesn't convert values behind your back.

while

let counter = 1;

while (counter <= 5) {
  console.log(counter);
  counter = counter + 1;
}

If you forget the line that adds one, the loop never ends and the tab freezes. If that happens, close it and open another.

for

for (let i = 1; i <= 5; i++) {
  console.log(i);
}

Inside the brackets there are three parts separated by ;:

1. Where it starts: let i = 1.
2. How long it keeps going: i <= 5.
3. How it moves on: i++, which is the same as i = i + 1.

Try it

for (let i = 1; i <= 10; i++) {
  console.log("3 x " + i + " = " + 3 * i);
}

3 x 1 = 3
3 x 2 = 6
…
3 x 10 = 30

Summary

• if, else if and else decide; while and for repeat.
• The condition goes in brackets and the block between curly braces.
• === compares; = stores.