() Converting values Number The, String () converts the string, Boolean () converts a Boolean value.
JavaScript data types
JavaScript There are five types of data values may comprise:
- String (string)
- Digital (number)
- Boolean (boolean)
- Object (object)
- Function (function)
There are three types of objects:
- Object (Object)
- Date (Date)
- Array (Array)
Meanwhile, there are two types of data can not contain the value:
- null
- undefined
typeof operator
You can use the typeof operator to determine the data type of a JavaScript variable.
Examples
typeof "Bill" // returns "String" typeof 3.14 // returns "Number" typeof NaN3 // returns "Number" typeof to false // returns "Boolean" typeof [1,2,3,4] // Returns "object" typeof {name: 'Bill', Age: 62 is} // returns "Object" typeof new new a Date () // returns "Object" typeof function () {} // return "function" typeof myCar //Return "undefined" * typeof null // returns "object"
caution:
- The data type is numerical NaN
- Data type of the array is the object
- Date data type is an object
- null data type is the object
- Data type is undefined undefined variables
- Data type has not been assigned a variable is undefined
You can not use typeof to determine whether the object is a JavaScript array (or date).
typeof data types
typeof operator is not a variable. It belongs to the operator. Operators (such as + - * /) no data types.
However, typeof always returns the string (containing the type operand).
constructor property
constructor property returns all the constructor function JavaScript variable.
Examples
"Bill".constructor // 返回 "function String() { [native code] }" (3.14).constructor // 返回 "function Number() { [native code] }" false.constructor // 返回 "function Boolean() { [native code] }" [1,2,3,4].constructor // 返回 "function Array() { [native code] }" {name:'Bill', age:62}.constructor // 返回" function Object() { [native code] }" new Date().constructor // 返回 "function Date() { [native code] }" function () {}.constructor // 返回 "function Function(){ [native code] }"
You can check to determine whether an object constructor property is an array (that contains the word "Array"):
Examples
function isArray(myArray) { return myArray.constructor.toString().indexOf("Array") > -1; }
Or, more simply, you can check whether the object is an array of functions:
Examples
function isArray(myArray) { return myArray.constructor === Array; }
You can check to determine whether an object constructor property to date (contains the word "Date"):
Examples
function isDate(myDate) { return myDate.constructor.toString().indexOf("Date") > -1; }
Or, more simply, you can check whether the object is a date function:
Examples
function isDate(myDate) { return myDate.constructor === Date; }
JavaScript type conversions
JavaScript variables can be converted into new variables, and another data type:
- Through the use of JavaScript function
- Automatic conversion by JavaScript itself
The value is converted to a string
Global method String () can be converted to a digital string.
It can be used in any type of numeric, text, variables or expressions:
Examples
String (X) // X Returns a string from numerical variables String (123) // return value from a text string 123 String (+ 23 is 100) // Returns a string from the numerical expression
Digital method toString () Similarly.
Examples
x.toString() (123).toString() (100 + 23).toString()
| method | description |
|---|---|
| toExponential() | Returns a string, rounding of numbers, and to write using exponential notation. |
| toFixed() | Returns a string, rounding of numbers, and uses the specified number of decimal places to write. |
| toPrecision() | Returns a string, the number written to the specified length. |
The Boolean is converted to a string
Global method String () can be converted into Boolean string.
String ( false ) // returns "false" String ( to true ) // returns "true"
Boolean method toString () empathy.
false .toString () // returns "false" to true .toString () // returns "true"
To convert a string date
Global method String () can be converted to the date string.
String (a Date ()) // returns "Fri Dec 13 2019 09:48:21 GMT + 0800 ( China Standard Time)"
Date method toString () empathy.
Examples
DATE (). ToString () // returns "Fri Dec 13 2019 09:48:21 GMT + 0800 ( China Standard Time)"
| method | description |
|---|---|
| getDate() | In order to obtain numerical count (1-31) of the Day |
| getDay() | In numerical terms or periphery (0-6) of |
| getFullYear() | Get four-digit years (yyyy) |
| getHours() | Obtaining (0-23) |
| getMilliseconds() | Ms is obtained (0-999) |
| getMinutes() | Get minutes (0-59) |
| getMonth() | Obtaining month (0-11) |
| getSeconds() | Get seconds (0-59) |
| getTime() | Get time (in milliseconds since January 1, 1970) |
Convert numeric strings
Global method Number () can be converted into a digital string.
String (such as "3.14") comprising a digital conversion to digital (such as 3.14).
0 is converted to an empty string.
Converted to other strings NaN (Not a number, not a number).
Number The ( "3.14") // Returns 3.14 Number The ( "") // returns 0 Number The ( "") // returns 0 Number The ( "99 88") // returns NaN
| method | description |
|---|---|
| parseFloat() | Parse the string and returns a float. |
| parseInt() | Parse the string and returns the integer. |
One yuan + operator
Unary + operator can be used to convert a digital variable:
Examples
var Y = ". 5"; // Y is a string var X = Y +; // X is a number
If you can not convert variable, it will become digital, but the value is NaN (Not a number):
Examples
var Y = "Bill"; // Y is a string var X = Y +; // X is a number (NaN)
Boolean value to convert
Global method Number () can also be converted to a digital Boolean.
Number The ( to false ) // Returns 0 Number The ( to true ) // Returns 1
The conversion date for digital
Global method Number () may be used to convert a digital date.
d = new Date(); Number The (D) // Returns 1576201701989
Date method getTime () empathy.
d = new Date(); d.getTime () // return 1576201701992, timestamp
Automatic type conversion
If JavaScript attempt to operate a kind of "wrong" data type, it will attempt to convert the value to the "right" type.
The results are not always what you expect:
5 + null // return null because 5 is converted into 0 "5" + null // Returns "5null" because null is converted to "null" , "5" + 2 // returns 52 because 2 is converted to "2" . " 5 "- 2 // return 3 as" 5 "is converted into 5 " 5 "*" 2 " // returned as 10" 5 "and" 2 "is converted into 2 and 5
Automatic string conversion
JavaScript automatically calls the variable toString () function, when you try to "output" object or variable:
document.getElementById("demo").innerHTML = myVar;
// If myVar =: {name "Fjohn" } // toString converted to "[Object Object]"
// if myVar = [1,2,3,4] // toString convert "1,2,3,4 "
// if myVar = new Date () // toString converted to" Fri Dec 13 2019 09:48:21 GMT + 0800 ( China standard time) "
Numbers and Boolean will be converted, but not obvious:
// If myVar = 123 // toString converted to "123" // if myVar = true // toString convert "to true" // if myVar = false // toString converted to "false"
JavaScript type conversion table
The following table lists the different JavaScript converted into digital values, and Boolean strings results:
| Original value | Converted to digital | Into a string | Convert logic |
|---|---|---|---|
| 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 |