Peng Old wet Recent monthly report mentioned valueof method, the interest came to the next ECMA5 in the ValueOf method of introduction, as follows:
15.2.4.4 Object.prototype.valueOf ()
When the valueof is called, the following steps are taken:
1. Let O is the result of calling Toobject passing, the this value as the argument.
2. If O is the result of calling the object constructor with a host Object (15.2.2.1), then
A. Return either O or another value such as the host object originally passed to the constructor. The specific result is implementation-defined.
3. Return O.
The explanation for valueof in the specification is very short, roughly: Call the Toobject method (an abstract method, as described later), and pass the value of this as a parameter.
for different parameters passed in when calling Toobject (this), the return values are asUnder
1, this is the host object, the return value depends on the browser's implementation, that is, the return of different browsers may be different (for host objects, refer to http://www.w3school.com.cn/js/pro_js_object_types.asp)
2, this is not a host object, returns the value of Toobject (this)
| Parameter type |
return results |
| Undefined |
Throw TypeError exception |
| Null |
Throw TypeError exception |
| Number |
Creates a number object that has an internal initial value of an incoming parameter value |
| String |
Creates a string object with an internal initial value of the parameter value passed in |
| Boolean |
Creates a Boolean object that has an internal initial value of the parameter value passed in |
| Object |
Returns the parameters passed in (no conversion) |
The following table is available based on the definition of Object.prototype.valueOf and the description of the abstract method Toobject
| OBJ type |
Object.prototype.valueOf.call (obj) returns the result |
| Undefined |
Throw TypeError exception |
| Null |
Throw TypeError exception |
| Number |
Object of number type with value equal to obj |
| String |
Object of type string with value equal to obj |
| Boolean |
A Boolean object with a value equal to obj |
| Object |
Obj object itself |
Give a few concrete examples:
Copy Code code as follows:
var num = 123;
Console.log (Num.valueof ()); Output: 123
Console.log (Num.valueof ()); Output: ' Number '
var unde = undefined;
Console.log (Object.prototype.valueOf.call (unde)); Output: ' Typeerror:cannot convert null to object '
var obj = {name: ' Casper '};
var linkobj = obj.valueof ();
Linkobj.name = ' Change ';
Console.log (Linkobj.name); Output: ' Change ' ... Description Obj.valueof () returns the object itself
In fact, there is no mention of an array or function object, and according to the following code, it can be assumed that when the Object.prototype.valueOf call, the argument is an array, a function-type object, and the returned result is the object itself:
Copy Code code as follows:
var arr = [1, 2, 3];
var Linkarr = arr.valueof ();
Linkarr[0] = [' Casper '];
Console.log (Linkarr); Output: [' Casper ', 2, 3]
var foo = function () {return 1;};
var linkfoo = foo.valueof ();
linkfoo.test = ' Casper ';
Console.log (linkfoo.test); Output: ' Casper '
After reading the above description, is there a kind of sudden feeling? If so, congratulations, maybe you and I have not fully understood.
As a simple example, when an object calling Object.prototype.valueOf is a numeric type, it is possible to declare that the object is named Num,num in the following two ways:
Copy Code code as follows:
var num = 123; Declares Console.log (typeof num) by object literal; Output: ' Number '
var num = new number (123); Declares Console.log (typeof num) by constructing a method; Output: ' object '
More abnormal declaration way, see "JS unknown five kinds of way to declare number"
For a description of the return value, the ECMA5 inside the text below：
Create a new Number object whose [[Primitivevalue]] Internal property is set to the value of the argument. 15.7 for a description of number objects.
According to the description of this text, it seems that num.valueof () should return a number object (that is, not a literal declaration), but in fact:
Copy Code code as follows:
var num = 123;
var tmp = num.valueof ();
Console.log (typeof tmp); Output: ' Number '
What the hell is going on here? So he looked at it carefully, and it seemed to be approaching the truth:
5.7.4.4 Number.prototype.valueOf ()
Returns this number value.
The valueof function is not generic; It throws a TypeError exception if it is not a number or a number object. Therefore, it cannot be transferred to the other kinds of the objects for use as a.
The original number has its own prototype valueof method, not directly inherited from the Object.prototype, similar, Boolean, String also has its own prototype valueof method, summed up as follows:
| Type |
Do you have a prototype valueof method that belongs to you? |
| Undefined |
No |
| Null |
No |
| Number |
There, Number.prototype.valueOf. |
| String |
There, String.prototype.valueOf. |
| Boolean |
There, Boolean.prototype.valueOf. |
| Object |
- |
In addition to this, Array, function does not have its own prototype valueof method, see specification Description:
Note The Array prototype object does not have a valueof property of its own; However, it inherits the ValueOf property from the standard built-in object prototype object.
The Function prototype object does not have a valueof property of its own; However, it inherits the ValueOf property from the object prototype object.
Add: Number.prototype.valueOf's internal conversion rules are a little more complicated than you think, and do not unfold here.
It's a chase, and now there are two questions to be wondered about.：
1. With regard to Toobject, when the parameter is a function object, the return object does not seem to have been clearly stated in the specification, it is currently only by experimental guessing (it may be I did not find it)
2.valueOf use of the scene, the actual development has not seen a brother used
Last and Last:
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