JavaScript Data Types
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JavaScript Data Types
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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";Try it Yourself »
Example
var x = "Volvo" + 16;Try it Yourself »
JavaScript evaluates expressions from left to right. Different sequences can
produce different results:
JavaScript:
var x = 16 + 4 + "Volvo";
Result:
20Volvo
Try it Yourself »
JavaScript:
var x = "Volvo" + 16 + 4;
Result:
Volvo164
Try it Yourself »
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.
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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
x = 5; // Now x is a Number
x = "John"; // Now x is a String
Try
it yourself »
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 quotesTry
it yourself »
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
Try
it yourself »
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
Try
it yourself »
Extra large or extra small numbers can be written with scientific
(exponential) notation:
Example
var y = 123e5; // 12300000
var z = 123e-5; // 0.00123Try
it yourself »
You will learn more about numbers later in this
tutorial.
JavaScript Booleans
Booleans can only have two values: true or false.
Example
var x = 5;
var y = 5;
var z = 6;
(x == y)
// Returns true
(x == z) // Returns
falseTry it Yourself »
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"];
Try it Yourself »
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";
Try it Yourself »
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"Try it Yourself »
Example
typeof 0 // Returns
"number"
typeof 314 // Returns
"number"
typeof 3.14 // Returns
"number"
typeof (3) // Returns
"number"
typeof (3 + 4) // Returns
"number"Try it Yourself »
Undefined
In JavaScript, a variable without a value, has the value undefined.
The typeof is also undefined.
Example
var car; // Value is undefined,
type is undefinedTry it Yourself »
Any variable can be emptied, by setting the value to undefined.
The type will also be undefined.
Example
car = undefined; // Value is undefined,
type is undefinedTry it Yourself »
Empty Values
An empty value has nothing to do with undefined.
An empty string has both a legal value and a type.
Example
var car = ""; //
The value is
"", the typeof is "string"Try it Yourself »
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 = firstName:"John", lastName:"Doe", age:50, eyeColor:"blue";
person = null; //
Now value is null,
but type is still an object
Try it Yourself »
You can also empty an object by setting it to undefined:
Example
var person = firstName:"John", lastName:"Doe", age:50, eyeColor:"blue";
person = undefined; //
Now both value and type is undefined
Try it Yourself »
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
// trueTry it Yourself »
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
Example
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)Try it Yourself »
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.
Example
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"
Try it Yourself »
The typeof operator returns "object" for arrays because in JavaScript arrays are objects.
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