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:
If the
variable cannot be converted, it will still become a number, but with the value NaN
(Not a Number):
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.
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.
The Date method toString() does the same.
Date Parts
Many Date Methods returns a Date part as a number.
You can use the toString() method to convert the return values to strings:
| Method | Description | Try it » |
|---|---|---|
| getFullYear() | Get the four digit year (yyyy) | Try it » |
| getMonth() | Get the month number (0-11) | Try it » |
| getDate() | Get the day number (1-31) | Try it » |
| getDay() | Get the weekday number (0-6) | Try it » |
| getHours() | Get the hours (0-23) | Try it » |
| getMinutes() | Get the minutes (0-23) | Try it » |
| getSeconds() | Get the seconds (0-59) | Try it » |
| getMilliseconds() | Get the milliseconds (0-999) | Try it » |
Month is Zero Based
By default, getMonth() returns an integer between 0 (January) and 11 (December).
To get a standard 1 to 12 range, you must add 1 to the result.
Month Name
Use the toLocaleString() method for easy formatting:
Learn More:
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.
