JavaScript Type Conversion

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JavaScript Data Types

Number() converts to a Number, String() converts to a String, Boolean() converts to a Boolean.

JavaScript Data Types

In JavaScript there are 5 different data types that can contain values:

string
number
boolean
object
function

There are 6 types of objects:

Object
Date
Array
String
Number
Boolean

And 2 data types that cannot contain values:

null
undefined

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The Typeof Operator

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

Example

typeof "John"                 // Returns "string"
typeof 3.14                   // Returns "number"
typeof NaN                    // Returns "number"
typeof false                  // Returns "boolean"
typeof [1,2,3,4]              // Returns "object"
typeof {name:'John', age:34}  // Returns "object"
typeof new Date()             // Returns "object"
typeof function () {}         // Returns "function"
typeof myCar                  // Returns "undefined" *
typeof null                   // Returns "object"

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Please observe:

The data type of NaN is number
The data type of an array is object
The data type of a date is object
The data type of null is object
The data type of an undefined variable is undefined *
The data type of a variable that has not been assigned a value is also undefined *

You cannot use typeof to determine if a JavaScript object is an array (or a date).

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The Data Type Of Typeof

The typeofoperator is not a variable. It is an operator. Operators ( + - * / ) do not have any data type.

But, the typeof operator always returns a string (containing the type of the operand).

The constructor Property

The constructor property returns the constructor function for all JavaScript variables.

Example

"John".constructor                // Returns function String() {[native code]}
(3.14).constructor                // Returns function Number() {[native code]}
false.constructor                 // Returns function Boolean() {[native code]}
[1,2,3,4].constructor             // Returns function Array()   {[native code]}
{name:'John',age:34}.constructor  // Returns function Object() {[native code]}
new Date().constructor            // Returns function Date() {[native code]}
function () {}.constructor        // Returns function Function(){[native code]}

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You can check the constructor property to find out if an object is an Array (contains the word "Array"):

Example

function isArray(myArray) {
return myArray.constructor.toString().indexOf("Array") > -1;
}

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Or even simpler, you can check if the object is an Array function:

Example

function isArray(myArray) {
return myArray.constructor === Array;
}

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You can check the constructor property to find out if an object is a Date (contains the word "Date"):

Example

function isDate(myDate) {
return myDate.constructor.toString().indexOf("Date") > -1;
}

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Or even simpler, you can check if the object is a Date function:

Example

function isDate(myDate) {
return myDate.constructor === Date;
}

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JavaScript Type Conversion

JavaScript variables can be converted to a new variable and another data type:

By the use of a JavaScript function
Automatically by JavaScript itself

Converting Numbers to Strings

The global method String() can convert numbers to strings.

It can be used on any type of numbers, literals, variables, or expressions:

Example

String(x)         // returns a string from a number variable x
String(123)       // returns a string from a number literal 123
String(100 + 23)  // returns a string from a number from an expression

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The Number method toString() does the same.

Example

x.toString()
(123).toString()
(100 + 23).toString()

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In the chapter Number Methods, you will find more methods that can be used to convert numbers to strings:

Method	Description
toExponential()	Returns a string, with a number rounded and written using exponential notation.
toFixed()	Returns a string, with a number rounded and written with a specified number of decimals.
toPrecision()	Returns a string, with a number written with a specified length

Converting Booleans to Strings

The global method String() can convert booleans to strings.

String(false) // returns "false"
String(true) // returns "true"

The Boolean method toString() does the same.

false.toString() // returns "false"
true.toString() // returns "true"

Converting Dates to Strings

The global method String() can convert dates to strings.

String(Date()) // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)"

The Date method toString() does the same.

Example

Date().toString() // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)"

In the chapter Date Methods, you will find more methods that can be used to convert dates to strings:

Method	Description
getDate()	Get the day as a number (1-31)
getDay()	Get the weekday a number (0-6)
getFullYear()	Get the four digit year (yyyy)
getHours()	Get the hour (0-23)
getMilliseconds()	Get the milliseconds (0-999)
getMinutes()	Get the minutes (0-59)
getMonth()	Get the month (0-11)
getSeconds()	Get the seconds (0-59)
getTime()	Get the time (milliseconds since January 1, 1970)

Converting Strings to Numbers

The global method Number() can convert strings to numbers.

Strings containing numbers (like "3.14") convert to numbers (like 3.14).

Empty strings convert to 0.

Anything else converts to NaN (Not a Number).

Number("3.14")    // returns 3.14
Number(" ")       // returns 0
Number("")        // returns 0
Number("99 88")   // returns NaN

Method	Description
parseFloat()	Parses a string and returns a floating point number
parseInt()	Parses a string and returns an integer

The Unary + Operator

The unary + operator can be used to convert a variable to a number:

Example

var y = "5"; // y is a string
var x = + y; // x is a number

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If the variable cannot be converted, it will still become a number, but with the value NaN (Not a Number):

Example

var y = "John"; // y is a string
var x = + y; // x is a number (NaN)

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Converting Booleans to Numbers

The global method Number() can also convert booleans to numbers.

Number(false) // returns 0
Number(true) // returns 1

Converting Dates to Numbers

The global method Number() can be used to convert dates to numbers.

d = new Date();
Number(d) // returns 1404568027739

The date method getTime() does the same.

d = new Date();
d.getTime() // returns 1404568027739

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Automatic Type Conversion

When JavaScript tries to operate on a "wrong" data type, it will try to convert the value to a "right" type.

The result is not always what you expect:

5 + null    // returns 5         because null is converted to 0
"5" + null  // returns "5null"   because null is converted to "null"
"5" + 2     // returns "52"      because 2 is converted to "2"
"5" - 2     // returns 3         because "5" is converted to 5
"5" * "2"   // returns 10        because "5" and "2" are converted to 5 and 2

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Automatic String Conversion

JavaScript automatically calls the variable's toString() function when you try to "output" an object or a variable:

document.getElementById("demo").innerHTML = myVar;

// if myVar = {name:"Fjohn"} // toString converts to "[object Object]"
// if myVar = [1,2,3,4] // toString converts to "1,2,3,4"
// if myVar = new Date() // toString converts to "Fri Jul 18 2014 09:08:55 GMT+0200"

Numbers and booleans are also converted, but this is not very visible:

// if myVar = 123             // toString converts to "123"
// if myVar = true            // toString converts to "true"
// if myVar = false           // toString converts to "false"

JavaScript Type Conversion Table

This table shows the result of converting different JavaScript values to Number, String, and Boolean:

Original Value	Converted to Number	Converted to String	Converted to Boolean
false	0	"false"	false
true	1	"true"	true
0	0	"0"	false
1	1	"1"	true
"0"	0	"0"	true
"000"	0	"000"	true
"1"	1	"1"	true
NaN	NaN	"NaN"	false
Infinity	Infinity	"Infinity"	true
-Infinity	-Infinity	"-Infinity"	true
""	0	""	false
"20"	20	"20"	true
"twenty"	NaN	"twenty"	true
[ ]	0	""	true
[20]	20	"20"	true
[10,20]	NaN	"10,20"	true
["twenty"]	NaN	"twenty"	true
["ten","twenty"]	NaN	"ten,twenty"	true
function(){}	NaN	"function(){}"	true
{ }	NaN	"[object Object]"	true
null	0	"null"	false
undefined	NaN	"undefined"	false

Values in quotes indicate string values.

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JavaScript Type Conversion

JavaScript Data Types

JavaScript Data Types

The Typeof Operator

Example

The Data Type Of Typeof

The constructor Property

Example

Example

Example

Example

Example

JavaScript Type Conversion

Converting Numbers to Strings

Example

Example

Converting Booleans to Strings

Converting Dates to Strings

Example

Converting Strings to Numbers

The Unary + Operator

Example

Example

Converting Booleans to Numbers

Converting Dates to Numbers

Automatic Type Conversion

Automatic String Conversion

JavaScript Type Conversion Table

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