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Hectal
Phase 1Beginner2 of 16 in JavaScript

Values, Types and Variables

Variables with let and const, the seven primitive types, how numbers and strings really work, type conversion, and == vs ===.

Every program is about values: numbers, words, yes/no answers, and "nothing". This phase teaches you how to keep values in variables (let and const, and why old var causes bugs), which types of value JavaScript has, and how to ask a value what type it is with typeof.

Then you'll go deep on the two types you'll use most. Numbers: why 0.1 + 0.2 isn't 0.3, what NaN and Infinity are, the largest safe whole number and when you need BigInt. Strings: quotes, escaping, template literals, length, indexing and why strings never change in place.

Finally you'll learn how JavaScript turns one type into another, on purpose and behind your back, and how to compare values safely with ===, Object.is and the truthy/falsy rules. These are the rules behind most beginner bugs and many interview questions. Every example is live: predict the output, press Run, and check.

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6 topics ~2 h 24 code blocks & diagrams
Start with the first topic
1
1.1

Variables: let, const and var

A variable is a name that points to a value so you can use it later; declare it with const by default, with let when the value must change, and avoid the old var.

25 min 4 code practice

2
1.2

Primitive Types

JavaScript has seven primitive types (string, number, boolean, null, undefined, bigint and symbol) plus objects; typeof tells you a value's type, with a few famous quirks.

19 min 4 code practice

3
1.3

Numbers in Depth

Every JavaScript number is a 64-bit floating-point value, which is why 0.1 + 0.2 isn't exactly 0.3, why NaN and Infinity exist, and why whole numbers above 2^53 − 1 need BigInt.

24 min 4 code practice

4
1.4

Strings Basics

A string is text: a fixed sequence of characters in quotes. You can write it three ways, escape special characters, insert values with template literals, read its length and single characters, but never change it in place.

23 min 4 code practice

5
1.5

Type Conversion

JavaScript converts values between types either when you ask (String(), Number(), Boolean(), parseInt, parseFloat) or automatically when an operator needs a different type; knowing the automatic rules prevents bugs like "5" + 1 giving "51".

24 min 4 code practice

6
1.6

Equality and Truthiness

Use === to compare values without hidden conversion, Object.is for the two cases === gets wrong (NaN and -0), and learn the eight falsy values, because every if converts its condition to true or false.

24 min 4 code practice

Practise this phase

Hands-on exercises: write the code, see your page live, and press Check to have it tested automatically.

1

Keep score with const and let

Make a tiny score keeper. Declare a constant playerName with the value "Asha" and a variable score that starts at 0.

Beginner
2

Write a type report

The starter declares five values. Fill each <span> with the type of the matching value, using typeof.

Beginner
3

Fix the cart total

A café's cart adds a coffee (1.10) and a muffin (2.20). The page shows 3.3000000000000003 and the check total === 3.3 logs false.

Beginner
4

A birthday card with a template literal

Using the variables in the starter, show this exact text in the card: Happy birthday, Ravi! You are 30 today.

Beginner
5

Fix a broken calculator

This calculator adds the two boxes when you press Add. Right now 2 and 3 give 23. Fix it so it adds numbers: 2 and 3 must show 5, and 1.5 and 2.25 must show 3.75.

Intermediate