javascript- type conversion

Type Conversion

JavaScript needs to be converted based on the type of their own

value	String	digital	Boolean value	Objects
undefined	"undefined"	NaN	false	throw TypeError
null	"null"	0	false	throw TypeError
true	"true"	1		new Boolean(true)
false	"false"	0		new Boolean(false)
"" (The empty string)		0	false	new String("")
"2.3"		2.3	true	new String("2.3")
"test"		NaN	true	new String("test")
0	"0"		false	new Number(0)
-0	"0"		false	new Number(-0)
NaN	"NaN"		false	new Number(NaN)
Infinity	"Infinity"		true	new Number(Infinity)
-Infinity	"-Infinity"		true	new Number(-Infinity)
1	"1"		true	new Number(1)
{}	See below the object into the original value	See below the object into the original value	true
[]	""	0	true
[1]	"1"	1	true
['a']	Use join () method	NaN	true
function(){}		NaN	true

In addition null, and undefinedany value other than both toString()methods, the results of this method and generally String()consistent with the method
All objects (including arrays and functions) for boolthe conversion are converted to true, for example: !!new Boolean(false), the result istrue

Object into the original value

A method!> And object to a String object to a digital conversion to be converted is done by calling the object. Javascript object has two different methods to perform the conversion.

All objects inherit two conversion methods.

The first is toString()that it's a reflection of the role is to return this object string.
JavaScript in the local object often defines its own toString()methods, such as:

class	`toString()`Method to realize	Examples
Ordinary objects	Return Type	`({a:1}).toString()` ==> `"[object Object]"`
Array type (Array class)	Converting each array element as a string, and a `,`connector	`[1,2,3].toString()` ==> `"1,2,3"`
Class of functions (Function class)	Return of this function implementation-defined representation, i.e. a user-defined function converts JavaScript source code string	`(function() {console.log(1)}).toString()` ==> `"function() {console.log(1)}"`
Date class (Date class)	Returns a readable (may be parsed Javascript) date and time strings	`(new Date(2019, 5, 31)).toString` ==> `"Mon Jul 01 2019 00:00:00 GMT+0800 (中国标准时间)"`
RegExp类(RegExp class)	Returns a regular expression amount of direct string	`/\d+/g.toString()` ==> `"/\d+/g"`

Another function converts the object is valueOf(). The task of this method is not defined in detail: If any of the original value exists, it will default object into a representation of its original value. The object is a composite value, and most objects can not be represented as a true original value, so the default valueOf()method simply returns the object itself, rather than a return to the original value.

Arrays, functions and regular expressions simply inherited this default method, calling Examples of these types of valueOf()method simply returns the object itself.
Date class defines the valueOf()method will return its internal representation: the number of milliseconds since January 1, 1970

(new Date(2019, 5, 31)).valueOf() // => 1561910400000

JavaScript objects to string conversion

If the object has a toString()method, this method is invoked. If it returns a primitive value, JavaScript this value to a string (if not itself a string), and returns the result string.
If the object does not toString()approach or this method does not return a primitive value, then the Javascript calls the valueOf()method. If this method exists, the JavaScript call it. If the return value is the original value, JavaScript will convert the value to a string (if itself is not a string) and returns the string result.
Otherwise, JavaScript can not be from toString()or valueOf()get a raw value, then he will throw a type error exception

JavaScript objects to digital conversion process

Objects to digital conversion process with the object to a string of similar conversion process, but he will first try to use the valueOf()method:

If the object has valueOf()a method, which returns an original value if, JavaScript then these converted into digital raw value (if necessary), and returns this number
Otherwise, if the object has toString()a method, which returns if a raw value, JavaScript and convert it to a digital return
Otherwise, throw an error exception type

Examples

Example: empty array into a digital 0

+[] // => 0

Example: the array contains a number of elements into a digital, the digital result is

+[100] // => 100

Explanation: Array inherits default valueOf()method, which returns an object rather than an original value, and therefore continue to call the toString()method; an empty string is converted to an empty array, then an empty string is converted to a digital 0. Similarly, comprising 100array into a string "100",
then converted to digital100

Example:

[] + [] // => '',在这个过程中先调用valueOf()方法，返回的数组的原始值还是一个数组，再调用toString()方法，即为："" + "" =""
[1,2] + 1 // => '1,21', [1,2] 先调用valueOf()，返回的还是数组本身，在调用toString()，返回字符串"1,2"，则 "1,2" + 1 ="1,21"
[1] + 1 // => 2
1 + {a:1} // => "1[object Object]"
{a:1} +1  // => 1, 这是因为js在解释代码时遇到{}会认为是一个代码块（代码区域），{a:1}已经结束的代码块，因此相当于原式=      +1   //  1
new Number(1) == 1 // => true

Two of the original operator +, if one of the operands is an object, a method using special JavaScript (target date to try to call toString(), and then tries to call valueOf(),
only to get the original value is used directly, without further converted into a number or string ; addition to the date the object, the first call attempt valueOf(), then the call toString(), the same will not be further converted into numeric or string)
which is converted to the original value calculation; ==, !=and relational operators Similarly, if an object with a comparison of the original value, the conversion to the original value conversion method performed in accordance with the object;
example:

let now = new Date();
now + 1 // "Fri May 31 2019 17:10:20 GMT+0800 (中国标准时间)1"
now - 1 // 1562034278023
now == now.toString() // true
now > (now - 1) // true

(!+[]+[]+![]).length // 9

(!(~+[]) + {})[--[~+""][+[]] * [~+[]] + ~~!+[]] + ({} + [])[[~!+[]] * ~+[]] // => sb

let a = {};
a.toString = () => 2
a.valueOf = () => 3
1 + a // 4
'1' + a // '13'