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JavaScript Data Types


Value type (basic type) : String, Number, Boolean, Null, Undefined, Symbol.

Reference data type : Object, Array, Function.

Note: Symbol is a new primitive data type introduced in ES6, representing unique values.


JavaScript Data Types

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

var length = 16;                               // Number
var lastName = "Johnson";                      // String
var cars = ["Mercedes", "Lamborghin", "BMW"];           // Array
var x = {firstName:"John", lastName:"dwayson"};    // 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 + "Lamborghin";

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

JavaScript will treat the example above as:

var x = "16" + "Lamborghin";

Example

var x = 16 + "Lamborghin";
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Example

var x = "Lamborghin" + 16;
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JavaScript evaluates expressions from left to right. Different sequences can produce different results:

JavaScript:

var x = 16 + 4 + "Lamborghin";

Result:

20Lamborghin
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JavaScript:

var x = "Lamborghin" + 16 + 4;

Result:

Lamborghin164
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In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Lamborghin".

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


JavaScript Has Dynamic Types

JavaScript has dynamic typing. This means that the same variable can be used as different types:

Example

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

JavaScript Strings

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

The string can be any text in quotes. You can use single or double quotes:

Example

var carName = "Lamborghin XC60";   // Using double quotes
var carName = 'Lamborghin XC60';   // Using single quotes

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

Example

var answer = "It's alright";             // Single quote inside double quotes
var answer = "He is called 'Johnny'";    // Single quotes inside double quotes
var answer = 'He is called "Johnny"';    // Double quotes inside single quotes
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You will learn more about strings later in this tutorial.


JavaScript Numbers

JavaScript has only one number type. Numbers can be with or without a decimal point:

Example

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

Very large or very small numbers can be written in scientific (exponential) notation:

Example

var y = 123e5;      // 12300000
var z = 123e-5;     // 0.00123
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You will learn more about numbers in the advanced part of this tutorial.


JavaScript Booleans

Boolean is often used in conditional tests. You will learn more about conditional testing later in this tutorial.

Example

var x = true;
var y = 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):

Example

var cars = ["Mercedes", "Lamborghin", "BMW"];
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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

Objects are separated by curly braces. Inside the parentheses, the properties of the object are defined as name and value pairs (name: value). The attributes are separated by commas:

Example

var person = {firstName:"John", lastName:"dwayson", age:50, eyeColor:"blue"};
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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:

Example

typeof "John"                // Returns string
typeof 3.14                  // Returns number
typeof false                 // Returns boolean
typeof [1,2,3,4]             // Returns object
typeof {name:'John', age:34} // Returns object
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Undefined

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

Example

var person;                  // Value is undefined, type is undefined
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Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

Example

person = undefined;          // Value is undefined, type is undefined
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Empty Values

An empty value has nothing to do with undefined.

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

Example

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

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

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

You can empty an object by setting it to null:

Example

var person = null;           // Value is null, but type is still an object
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You can also empty an object by setting it to undefined:

Example

var person = undefined;     // Value is undefined, type is undefined
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Difference Between Undefined and Null

Undefined This value indicates that the variable contains no value. You can empty a variable by setting its value to null.
typeof undefined             // undefined
typeof null                  // object
null === undefined           // false
null == undefined            // true
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