问题
In this question about the passing of arguments in JavaScript functions, we learn that everything is passed by value in JavaScript.
In Mozilla documents, it is mentioned that the primitive types are immutable, and objects are. Although I came from the procedural and structured programming school, I was able to quickly pick up the concepts.
In the ECMAScript standard, it is defined that "An Object is 'logically' a collection of properties". The standard also defines how objects may be compared, but left out on what happens when an object goes through the GetValue() pseudo-function that converts references into values.
So, I gave an answer in the question basically saying that this area had been left undefined.
My Question
I feel that by "left undefined", I meant, it wasn't philosophically thoroughly clear, what the value of an Object is. The standard had gone through a few revisions, and its size is ever increasing.
In short, an object is a collection, but what is the value of the collection? Is it the makeup of its content? Or is it individuality? Or have I been missing out on some important texts?
回答1:
In the ECMAScript spec, every object is defined to have certain 'internal methods', some of which (e.g., [[DefineOwnProperty]] and [[Put]]) can change the state of the object. Ultimately, the mutability of objects is defined via the use of such internal methods.
回答2:
GetValue() doesn't leave out what happens to objects -- step #1 is:
If Type(V) is not Reference, return V.
So it you pass it an object, you get back the same object.
(Which refutes one of your premises, but I'm not sure it resolves your question.)
回答3:
See section 4.3.26 "property" of the 5.1 edition. The note says:
Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or a function object) or indirectly by a pair of accessor functions.
We can take this as meaning a data value is one of the following:
- Primitive Value: such as C language double, _Bool, ((void*)0), etc.
- An object: which can be interpreted as a special C language structure containing the underlaying information about the object.
- Function object: which is just a special case of 2, possibly the result of JIT compilation.
The reason this note for the definition of property is important, is because, everything - even function block scopes - are objects (or at least described in terms of one). Therefore, if we can determine that "the value of an object" is its individuality rather than its content makeup, then with the fact that every object accessible from a JavaScript program, is accessed as if its the property of some other object.
In section 4.2 "Language Overview", it says:
A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, and String; an object is a member of the remaining built-in type Object; and a function is a callable object.
Although this is an informal section, it can be seen that an object differs from a primitive value in a significant way.
As an interpretation, let's consider the value of an object is the object itself as we can infer from the "GetValue()" pseudo function - the overview says "an object is a member of the ... type Object - therefore, the value is the membership to the Object type.
To use a physics analogy to explain the relationship between membership and individuality, we see too electrons. They are identical in content, they're both the members of the Universe, yet they are two different individuals.
Therefore, we infer that - the value of a JavaScript Object, is its individuality.
Finally as to the question as asked in the title.
The mutibility of individual objects is defined in terms of a series of specificational pseudo functions, and immutability of other types is defined using the definition of value membership of types and specification pseudo functions operating on the primitive type values.
来源：https://stackoverflow.com/questions/45388408/where-is-the-mutability-of-objects-defined-in-ecmascript