JavaScript Type Conversion

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JavaScript Type Conversion
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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"
»
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] }"
»
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;
}
»

Or even simpler, you can check if the object is an Array function:

Example
function isArray(myArray) {
    return myArray.constructor === Array;
}
»

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;
}
»

Or even simpler, you can check if the object is a Date function:

Example
function isDate(myDate) {
    return myDate.constructor === Date;
}
»

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
»
The Number method toString() does the same.

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


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