JavaScript data types

Author: MDBootstrap

JavaScript Data Types

JavaScript variables can hold many data types: numbers, strings, objects and more:

                      var length = 16;                               // Number
                      var lastName = "Johnson";                      // String
                      var x = {firstName:"John", lastName:"Doe"};    // Object

The Concept of Data Types

In programming, data types is an important concept.

To be able to operate on variables, it is important to know something about the type.

Without data types, a computer cannot safely solve this:

                      var x = 16 + "Volvo";

Does it make any sense to add "Volvo" to sixteen? Will it produce an error or will it produce a result?

JavaScript will treat the example above as:

                    var x = "16" + "Volvo";

Remember: When adding a number and a string, JavaScript will treat the number as a string.

JavaScript evaluates expressions from left to right. Different sequences can produce different results:

                      var x = 16 + 4 + "Volvo";  // Result 20Volvo

                      var x = "Volvo" + 16 + 4;    // Result Volvo164

In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".

In the second example, since the first operand is a string, all operands are treated as strings.

JavaScript Types are Dynamic

JavaScript has dynamic types. This means that the same variable can be used to hold different data types:

                      var x;           // Now x is undefined
                      x = 5;           // Now x is a Number
                      x = "John";      // Now x is a String

JavaScript Strings

A string (or a text string) is a series of characters like "John Doe".

Strings are written with quotes. You can use single or double quotes:

                    var carName1 = "Volvo XC60";   // Using double quotes
                    var carName2 = 'Volvo XC60';   // Using single quotes

You can use quotes inside a string, as long as they don't match the quotes surrounding the string:

                      var answer1 = "It's alright";             // Single quote inside double quotes
                      var answer2 = "He is called 'Johnny'";    // Single quotes inside double quotes
                      var answer3 = 'He is called "Johnny"';    // Double quotes inside single quotes

JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:

                      var x1 = 34.00;     // Written with decimals
                      var x2 = 34;        // Written without decimals

JavaScript Booleans

Booleans can only have two values: true or false.

                      var x = 5;
                      var y = 5;
                      var z = 6;
                      (x == y)       // Returns true
                      (x == z)       // Returns false

Booleans are often used in conditional testing.

You will learn more about conditional testing later in this tutorial.

JavaScript Arrays

JavaScript arrays are written with square brackets.

Array items are separated by commas.

The following code declares (creates) an array called cars, containing three items (car names):

                      var cars = ["Saab", "Volvo", "BMW"];

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

You will learn more about arrays later in this tutorial.

JavaScript Objects

JavaScript objects are written with curly braces {}.

Object properties are written as name:value pairs, separated by commas.

                      var person = {
                        firstName : "John",
                        lastName  : "Doe",
                        age     : 50,
                        eyeColor  : "blue"

The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

You will learn more about objects later in this tutorial.

The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable.

                      typeof ""             // Returns "string"
                      typeof "John"         // Returns "string"
                      typeof "John Doe"     // Returns "string"
                      typeof 0              // Returns "number"
                      typeof 314            // Returns "number"
                      typeof 3.14           // Returns "number"
                      typeof (3)            // Returns "number"
                      typeof (3 + 4)        // Returns "number"

                      x = "2";
                      alert(typeof x);   // Returns "string"


In JavaScript, a variable without a value, has the value undefined. The type is also undefined.

                      var car;    // Value is undefined, type is undefined

Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

                      var car = "Volvo";
                      car = undefined;  // Value is undefined, type is undefined

Empty Values

An empty value has nothing to do with undefined.

An empty string has both a legal value and a type.

                      var car = "";    // The value is "", the typeof is "string"


In JavaScript null is "nothing". It is supposed to be something that doesn't exist.

Unfortunately, in JavaScript, the data type of null is an object.

Note: You can consider it's a bug in JavaScript that typeof null is an object. It should be null.

You can empty an object by setting it to null:

                      var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
                      person = null;    // Now value is null, but type is still an object

You can also empty an object by setting it to undefined:

                      var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
                      person = undefined;   // Now both value and type is undefined

Difference Between Undefined and Null

undefined and null are equal in value but different in type:

                      typeof undefined           // undefined
                      typeof null                // object
                      null === undefined         // false
                      null == undefined          // true

Primitive Data

A primitive data value is a single simple data value with no additional properties and methods.

The typeof operator can return one of these primitive types:

  • string
  • number
  • boolean
  • undefined

                      typeof "John"              // Returns "string" 
                      typeof 3.14                // Returns "number"
                      typeof true                // Returns "boolean"
                      typeof false               // Returns "boolean"
                      typeof x                   // Returns "undefined" (if x has no value)

Complex Data

The typeof operator can return one of two complex types:

  • function
  • object

The typeof operator returns object for both objects, arrays, and null.

The typeof operator does not return object for functions.

                      typeof {name:'John', age:34} // Returns "object"
                      typeof [1,2,3,4]             // Returns "object" (not "array", see note below)
                      typeof null                  // Returns "object"
                      typeof function myFunc(){}   // Returns "function"

The typeof operator returns "object" for arrays because in JavaScript arrays are objects.

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