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JavaScript Type Conversion


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 3 types of objects:

  • Object
  • Date
  • Array

And 2 data types that cannot contain values:

  • null
  • undefined

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 (if myCar is not declared)
typeof null                   // Returns object

Try it yourself »

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

You cannot use typeof to define if an object is an JavaScript Array or a JavaScript Date.


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] }

Try it yourself »

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;
}

Try it yourself »

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;
}

Try it yourself »


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

Try it yourself »

The Number method toString() does the same.

Example

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

Try it yourself »

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

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

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


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


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" + 1     // returns "51"      because 1 is converted to "1" 
"5" - 1     // returns 4         because "5" is converted to 5


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"



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