Data Types in JavaScript

Data types specify what kind of data can be stored and manipulated within a program.

There are six basic data types in JavaScript which can be divided into three main categories: primitive, composite, and special data types. String, Number, and Boolean are primitive data types. Object, Array, and Function (which are all types of objects) are composite data types. Whereas Undefined and Null are special data types.

Primitive data types can hold only one value at a time, whereas composite data types can hold collections of values and more complex entities.

The String Data Type

The string data type is used to represent textual data (i.e. sequences of characters). Strings are created using single or double quotes surrounding one or more characters, as shown below:

var a = 'Hi there!';  // using single quotes
var b = "Hi there!";  // using double quotes
var a = "Let's have a cup of coffee."; // single quote inside double quotes
var b = 'He said "Hello" and left.';  // double quotes inside single quotes
var c = 'We\'ll never give up.';     // escaping single quote with backslash

The Number Data Type

The number data type is used to represent positive or negative numbers with or without decimal place, or numbers written using exponential notation e.g. 1.5e-4 (equivalent to 1.5x10-4).

var a = 25;         // integer
var b = 80.5;       // floating-point number
var c = 4.25e+6;    // exponential notation, same as 4.25e6 or 4250000
var d = 4.25e-6;    // exponential notation, same as 0.00000425

The Number data type also includes some special values which are: Infinity, -Infinity and NaN. Infinity represents the mathematical Infinity ∞, which is greater than any number. Infinity is the result of dividing a nonzero number by 0.

alert(16 / 0);  // Output: Infinity
alert(-16 / 0); // Output: -Infinity
alert(16 / -0); // Output: -Infinity

While NaN represents a special Not-a-Number value. It is a result of an invalid or an undefined mathematical operation, like taking the square root of -1 or dividing 0 by 0, etc.

alert("Some text" / 2);       // Output: NaN
alert("Some text" / 2 + 10);  // Output: NaN
alert(Math.sqrt(-1));         // Output: NaN

The Boolean Data Type

The Boolean data type can hold only two values: true or false. It is typically used to store values like yes (true) or no (false), on (true) or off (false), etc.

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

Boolean values also come as a result of comparisons in a program. The following example compares two variables and shows the result in an alert dialog box:

var a = 2, b = 5, c = 10;

alert(b > a) // Output: true
alert(b > c) // Output: false

The Undefined Data Type

The undefined data type can only have one value-the special value undefined. If a variable has been declared, but has not been assigned a value, has the value undefined.

var a;
var b = "Hello World!"

alert(a) // Output: undefined
alert(b) // Output: Hello World!

The Null Data Type

This is another special data type that can have only one value-the null value. A null value means that there is no value. It is not equivalent to an empty string ("") or 0, it is simply nothing.

A variable can be explicitly emptied of its current contents by assigning it the null value.

var a = null;
alert(a); // Output: null

var b = "Hello World!"
alert(b); // Output: Hello World!

b = null;
alert(b) // Output: null

The Object Data Type

The object is a complex data type that allows you to store collections of data.

An object contains properties, defined as a key-value pair. A property key (name) is always a string, but the value can be any data type, like strings, numbers, booleans, or complex data types like arrays, function and other objects.

var emptyObject = {};
var person = {"name": "Clark", "surname": "Kent", "age": "36"};
var car = {
    "model": "BMW X3",
    "color": "white",
    "doors": 5
}

The Array Data Type

An array is a type of object used for storing multiple values in single variable. Each value (also called an element) in an array has a numeric position, known as its index, and it may contain data of any data type-numbers, strings, booleans, functions, objects, and even other arrays. The array index starts from 0.

The simplest way to create an array is by specifying the array elements as a comma-separated list enclosed by square brackets.

var colors = ["Red", "Yellow", "Green", "Orange"];
var cities = ["London", "Paris", "New York"];

alert(colors[0]);   // Output: Red
alert(cities[2]);   // Output: New York

The Function Data Type

The function is callable object that executes a block of code. Since functions are objects, so it is possible to assign them to variables.

var greeting = function(){ 
    return "Hello World!"; 
}

alert(greeting());     // Output: Hello World!

In fact, functions can be used at any place any other value can be used. Functions can be stored in variables, objects, and arrays. Functions can be passed as arguments to other functions, and functions can be returned from functions.