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Tutorial

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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 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

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:

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

EXAMPLE ❯

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:

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

The Number method toString() does the same.

EXAMPLE ❯

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.

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:

EXAMPLE ❯

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.

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

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