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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" *
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 *
  • 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).


The Data Type of typeof

The typeof operator 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] }"
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

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
Try it Yourself »

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)
Try it Yourself »

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" + 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
Try it Yourself »

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
Try it
false 0 "false" false Try it »
true 1 "true" true Try it »
0 0 "0" false Try it »
1 1 "1" true Try it »
"0" 0 "0" true Try it »
"000" 0 "000" true Try it »
"1" 1 "1" true Try it »
NaN NaN "NaN" false Try it »
Infinity Infinity "Infinity" true Try it »
-Infinity -Infinity "-Infinity" true Try it »
"" 0 "" false Try it »
"20" 20 "20" true Try it »
"twenty" NaN "twenty" true Try it »
[ ] 0 "" true Try it »
[20] 20 "20" true Try it »
[10,20] NaN "10,20" true Try it »
["twenty"] NaN "twenty" true Try it »
["ten","twenty"] NaN "ten,twenty" true Try it »
function(){} NaN "function(){}" true Try it »
{ } NaN "[object Object]" true Try it »
null 0 "null" false Try it »
undefined NaN "undefined" false Try it »

Values in quotes indicate string values.

Red values indicate values (some) programmers might not expect.