Values: Types, Operators and Math

Mission 4, step 3. Programs work with values: names, numbers, yes and no. Today you learn the kinds of values and how JavaScript calculates with them.

Quick check

Quick recall

10 XP

Variables

A variable is a box with a name for a value. let makes a box whose value can change. const makes a box that keeps its value. Older code uses var; you do not need it.

let age = 25;         // can change: age = 26;
const name = "Ayla";  // stays "Ayla"

Types of values

The type is the kind of a value. The operator typeof tells you the type: typeof 5 gives "number".

Type	Examples	Meaning
string	"Ayla", "0000 000"	Text in quotes, also digits in quotes
number	25, 3.14, -5	A number without quotes
boolean	true, false	Yes or no
undefined	let box;	No value yet
null	let wallet = null;	Empty on purpose
object	{ name: "Ayla", age: 25 }	Named values in one box; read one with person.name

Good to know: typeof null gives "object". It is an old mistake in JavaScript that was never changed.

Grouping

Which type?

Required 15 XP

Operators

• + - * / work as in maths.
• % gives the remainder: 10 % 3 is 1. An even number has remainder 0 when you divide it by 2.
• ++ adds 1, -- takes 1 away.
• score += 10 means score = score + 10; -= takes away.

let score = 0;   // create it first
score += 10;     // 10
score++;         // 11

The slide uses score += 10 without creating score, which stops with an error. With text, + joins:

console.log("Lena" + " " + "Weber");   // Lena Weber
console.log("5" + 5);                  // 55: one value is text

Quick check

Text plus number

Required 10 XP

Now be a type detective. This exercise counts for the Console Detective badge.

Practice

Type detective

Required 30 XP

Find out the type of four values with typeof. Then join the city and the number in one sentence.

The Math object

Code	Result
Math.round(4.6) (nearest whole number)	5
Math.ceil(4.1) (always up) and Math.floor(4.9) (always down)	5 and 4
Math.min(10, 5, 20, -3) and Math.max(10, 5, 20)	-3 and 20
Math.abs(-50) (without the minus)	50
Math.pow(2, 3) and Math.sqrt(16)	8 and 4
Math.random()	A number from 0 up to 1, never 1, for example 0.8234

Worked example: the hypotenuse

The slides use Pythagoras: in a triangle with a right angle, a² + b² = c². With a = 3 and b = 4, c is 5.

The slide's code misses one thing in step 2. Run it, read the message, fix it. This counts for the Bug Hunter badge.

Practice

Slide bug: the missing hypotenuse

Required 30 XP

This code comes from a course slide. It has a small bug. Find it and fix it.

Variable

A box with a name for a value.

Type

The kind of a value: string, number, boolean, undefined, null or object.

typeof

An operator that gives the type as text.

Operator

A sign that works with values, like +, % or +=.

In short

• let for values that change, const for values that stay.
• typeof names the type of a value.
• % gives the remainder, += changes a value, + joins text.
• Math rounds and calculates; Math.random() makes random numbers.

Sources

1. MDN Web Docs (2026). Storing the information you need: Variables.
2. MDN Web Docs (2026). JavaScript data types and data structures and typeof.
3. MDN Web Docs (2026). Basic math in JavaScript: numbers and operators and Remainder (%).
4. MDN Web Docs (2026). Math.