Data types

Why it matters

Values come in different types: text, numbers, "yes/no". The type decides what you can do with a value. 5 + 5 is 10, but "5" + "5" is "55". Data from forms, files and the internet almost always arrives as strings, and you must be able to turn them into numbers — otherwise your calculator will join digits instead of adding them.

The main types

Type	Examples	What it is
string	"Anna", '42', `hello`	a string, text
number	42, 3.14, -7	a number (integers and decimals are one type)
boolean	true, false	logical "yes / no"
undefined	undefined	"no value yet"
null	null	"no value, on purpose"

There are also bigint and symbol — they're rarely needed. Arrays and objects come later.

Finding the type: typeof

console.log(typeof "hello");
console.log(typeof 42);
console.log(typeof true);
console.log(typeof undefined);

Output:

string
number
boolean
undefined

A quirk of the language: typeof null gives "object". It's an old bug that can no longer be fixed.

undefined and null

If you create a variable and put nothing in it, it holds undefined:

let answer;
console.log(answer);

Output:

undefined

null is set by you when you want to say "it's empty here": let winner = null; — no winner yet.

Template strings

A string in backticks ` is called a template string. You can insert a value into it with ${...}:

const name = "Anna";
const age = 15;
console.log(`My name is ${name}, I am ${age}`);
console.log(`Next year I'll be ${age + 1}`);

Output:

My name is Anna, I am 15
Next year I'll be 16

Any expression can go inside ${}. It's more convenient than joining pieces with +.

Type conversion

To a number:

console.log(Number("42"));      // 42
console.log(Number("3.5"));     // 3.5
console.log(Number(""));        // 0
console.log(Number("12px"));    // NaN
console.log(parseInt("12px"));  // 12
console.log(parseFloat("1.5kg")); // 1.5

NaN (Not a Number) is a special numeric value: the result of a failed conversion. You check for it with Number.isNaN(x). A normal comparison doesn't work: NaN === NaN is false!

Number requires the whole string to be a number. parseInt and parseFloat read digits from the start of the string and stop at the first extra character.

To a string: String(42) → "42", or `${42}`.

To a boolean: Boolean(x). Values that turn into false are called "falsy": 0, "" (empty string), null, undefined, NaN, false. Everything else is true, even the string "0" and the string "false".

Automatic conversion

JavaScript sometimes changes types by itself — and that's a source of surprises:

console.log("5" + 2);   // 52  — plus with a string joins
console.log("5" - 2);   // 3   — minus turns the string into a number
console.log("5" * "2"); // 10
console.log(true + 1);  // 2   — true becomes 1

Output:

52
3
10
2

Rule: if either side of + is a string, it joins. So it's better to convert form data to numbers explicitly with Number(...).

How it works inside

Every value in memory is stored together with a label of its type. When you write a + b, the engine first looks at the types: two numbers — it adds them; a string — it turns the other part into a string and joins. Types are not tied to a variable: the same let can hold first a number and then a string. Such a language is called dynamically typed.

Common mistakes

1. Joining instead of adding. "10" + 5 → "105". Fix: Number("10") + 5.
2. Number on a string with letters. Number("12px") → NaN. You need parseInt("12px").
3. Comparing with NaN. x === NaN is always false. Use Number.isNaN(x).
4. Normal quotes instead of backticks. "Hello, ${name}" prints the text as is, with the dollar and braces. Insertion only works inside `…`.
5. The string "false" is truthy. Boolean("false") → true, because the string isn't empty.

Cheat sheet

What	How	Result
Get the type	typeof x	"string", "number"…
String → number	Number("42")	42
Number from the start	parseInt("12px")	12
Decimal from the start	parseFloat("1.5kg")	1.5
Check for NaN	Number.isNaN(x)	true / false
Number → string	String(42)	"42"
Insertion	`Total: ${sum}`	"Total: 12"
Falsy values	0 "" null undefined NaN false	everything else is truthy