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
- Object
- Date
- Array
- 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" - 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]} Example
function isArray(myArray) {
return myArray.constructor.toString().indexOf("Array") > -1;
} Example
function isArray(myArray) {
return myArray.constructor === Array;
} Example
function isDate(myDate) {
return myDate.constructor.toString().indexOf("Date") > -1;
} 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 Example
x.toString()
(123).toString()
(100 + 23).toString() Converting Booleans to Strings
The global method String() can convert booleans to strings.String(false) // returns "false"String(true) // returns "true" 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)" Example
Date().toString() // returns "Thu Jul 17 2014 15:38:19 GMT+0200 (W. Europe Daylight Time)" 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 3.14Number(" ") // returns 0 Number("") // returns 0Number("99 88") // returns NaN The Unary + Operator
The unary + operator can be used to convert a variable to a number:Example
var y = "5"; // y is a stringvar x = + y; // x is a number Example
var y = "John"; // y is a stringvar x = + y; // x is a number (NaN) Converting Booleans to Numbers
The global method Number() can also convert booleans to numbers.Number(false) // returns 0Number(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 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" // 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:| 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 | |||
| 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) | |||
| Method | Description | |||
| parseFloat() | Parses a string and returns a floating point number | |||
| parseInt() | Parses a string and returns an integer | |||
| Original Value | Converted to Number | Converted to String | Converted to Boolean | Try it |
| 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 |
