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JavaScript Type Conversion

  • Converting Strings to Numbers
  • Converting Numbers to Strings
  • Converting Dates to Numbers
  • Converting Numbers to Dates
  • Converting Booleans to Numbers
  • Converting Numbers to Booleans

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 Strings to Numbers

The global method Number() converts a variable (or a value) into a number.

A numeric string (like "3.14") converts to a number (like 3.14).

An empty string (like "") converts to 0.

A non numeric string (like "John") converts to NaN (Not a Number).

Examples

These will convert:

Number("3.14")
Number(Math.PI)
Number(" ")
Number("")

These will not convert:

Number("99 88")
Number("John")
Try it Yourself »

Number Methods

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

Method Description
Number() Returns a number, converted from its argument
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

let y = "5";      // y is a string
let 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

let y = "John";   // y is a string
let x = + y;      // x is a number (NaN)
Try it Yourself »


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 »

More Methods

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 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

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 Booleans to Numbers

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

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

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"

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.