Learn JS #31

JavaScript Type Conversion

Convertings:
  • 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() 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

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

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)

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

The Number method toString() does the same.

Example

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

More Methods

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)"

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

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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