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JavaScript Data Types


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";

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

Example

var x = 16 + "Volvo";
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Example

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

JavaScript:

var x = 16 + 4 + "Volvo";

Result:

20Volvo
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JavaScript:

var x = "Volvo" + 16 + 4;

Result:

Volvo164
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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:

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 Doe".

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

Example

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

You can use quotes inside a string, 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 type of numbers.

Numbers can be written with, or without decimals:

Example

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

Extra large or extra small numbers can be written with scientific (exponential) notation:

Example

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


JavaScript Booleans

Booleans can only have two values: true or false.

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 = ["Saab", "Volvo", "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

JavaScript objects are written with curly braces.

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

Example

var person = {firstName:"John", lastName:"Doe", 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.

The typeof operator returns the type of a variable or an expression:

Example

typeof ""                  // Returns "string"
typeof "John"              // Returns "string"
typeof "John Doe"          // Returns "string"
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Example

typeof 0                   // Returns "number"
typeof 314                 // Returns "number"
typeof 3.14                // Returns "number"
typeof (3)                 // Returns "number"
typeof (3 + 4)             // Returns "number"
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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
  • null
  • undefined

Example

typeof "John"              // Returns "string"
typeof 3.14                // Returns "number"
typeof true                // Returns "boolean"
typeof false               // Returns "boolean"
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Complex Data

The typeof operator can return one of two complex types:

  • function
  • object

Example

typeof [1,2,3,4]             // Returns "object" (not "array", see note below)
typeof {name:'John', age:34} // Returns "object"
typeof function myFunc(){}   // Returns "function"
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The typeof operator returns "object" for arrays because in JavaScript arrays are objects.


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 doesn't exist.

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

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

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

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