Understanding Data Types in JavaScript

Data Types specify what kind of data can be stored and manipulated within a program. A value in JavaScript is always of a certain type. For example, a string, a boolean or a number.

Primitive Types

In JavaScript, a primitive type is a data that is not an object and has no methods. All primitives are immutable, they cannot be altered. Variables can be reassigned a new value, but the existing value can not be changed in the ways that objects, arrays, and function can. There are 7 primitive data types: string, number, bigint, boolean, undefined, null and symbol.

String

const str = "Single quotes string";
const str2 = 'Double quotes string';
const phrase = `Backticks string ${str} ${str2}`

The string data type is used to represent a sequence of characters. strings are useful for holding data that can be represented in text form.

They can be represented with single quotes, double quotes and backticks. There is no difference between single and double quotes.

Backticks are “extended functionality” quotes. They allow us to embed variables and expressions into a string by wrapping them in ${…}, for example:

const firstName = "Matheus";
const lastName = "Silva";

console.log(`Helo, my name is ${firstName} ${lastName}`)

// Helo, my name is Matheus Silva

Numbers

const a = 100;
const b = 55.5;

The number data type is used to represent positive or negative numbers, with or without decimal place.

There are many operations for numbers, e.g. multiplication '*', division '/', addition '+', subtraction '-', and so on.

Besides regular numbers, there are so-called “special numeric values” which also belong to this data type: Infinity, -Infinity and NaN.

const infinity = 1 / 0; // Infinity
const negInfinity = -1 / 0; // -Infinity
const notANumber = 'Hello' / 2; // NaN

BigInt

The BigInt represents integers of arbitrary length.

In JavaScript, the “number” type cannot represent integer values larger than (253-1) (that’s 9007199254740991), or less than -(253-1) for negatives. It’s a technical limitation caused by their internal representation.

A BigInt value is created by appending n to the end of an integer:

const bigInt = 1234567890123456789012345678901234567890n;

Boolean

The boolean data type has only two values: true or false.

This type is commonly used to store yes/no values: true means “yes, correct”, and false means “no, incorrect”.

const isReading = true;   // yes, I'm reading
const isSleeping = false; // no, I'm not sleeping

Boolean values also come as a result of comparisons.

const a = 2, b = 5, c = 10;
 
console.log(b > a) // true
console.log(b > c) // false

Undefined

The undefined type can only have one value, which is a special value undefined. One example is when a variable is declared but it has not been assigned a value yet.

const a;
 
console.log(a) // undefined

Null

The null type is also a special value, which can only be null. A null value means that there is no value. It is not equivalent to an empty string ("") or 0, it is simply nothing.

let name = null;

Symbols

The symbol type is used to create unique identifiers for objects. We will get deeper into symbols in the object section.

Summary

There are 8 basic data types in JavaScript.

• number for numbers of any kind: integer or floating-point, integers are limited by ±(253-1).
• bigint is for integer numbers of arbitrary length.
• string for strings. A string may have zero or more characters, there’s no separate single-character type.
• boolean for true/false.
• null for unknown values – a standalone type that has a single value null.
• undefined for unassigned values – a standalone type that has a single value undefined.
• symbol for unique identifiers.
• object for more complex data structures.

In the next chapter, we will be talking about Objects, also referred as Non-primitive or Reference Data Type.