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 (if myCar is not declared)
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
 |
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.
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:
If the variable cannot be converted, it will still become a number, but with the value NaN (Not a number):
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 » |
| "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.
