On: Deep-copying in JavaScript

[chasingtheflow]

It is for objects, just not for primitives.

[williamdclt → chasingtheflow]

No. Object are passed by value. Exact same example apply:

    let a = {key: 'a'};
    
    function foo( a ) {
      a = {otherKey: 'b'};
    }
    
    foo( a );
    
    console.log( a ); // {key: 'a'}

[MJSplot_author → williamdclt]

But it is also true that:

    let a = {key: 'a'};
    
    function foo( a ) {
      a['otherKey'] = 'b';
    }
    
    foo( a );
    
    console.log( a ); // {key: 'a',otherKey: 'b'}

[williamdclt → MJSplot_author]

Yes, because "a" is not an object, it is a reference to an object. This reference is passed by value. This kind of thing is why I'd recommand everybody to know C: when you know pointers there's no magic anywhere anymore

[teamonkey → williamdclt]

Could you give an example in C of how the above works?

[pjmlp → teamonkey]

Something like this, without any kind of error checking being done.

    #include <stdlib.h>

    // assume some hash table library, exercise for the reader
    typedef struct{} *hashtable_t;
    extern hashtable_t hashtable_init(void);
    extern void hashtable_put(hashtable_t hashtable, const char* key, const char* value);
    extern void hashtable_dump(hashtable_t hashtable);

    typedef struct {
      hashtable_t table;
    } data_t;

    void foo(data_t* a)
    {
        hashtable_put(a->table, "otherKey", "b");
    }

    int main(void)
    {
        // let a = {key: 'a'};
        data_t* a = (data_t*) malloc(sizeof(data_t));
        a->table = hashtable_init();
        hashtable_put(a->table, "key", "a");

        foo(a);

        // console.log( a );
        hashtable_dump(a->table);
        return 0;
    }

The function foo() gets the numeric value of the a pointer, thus a parameter inside foo() points to the same memory location.

If C had pass-by-reference, it would be possible to give (implicitly) the memory location of the local variable a in main() instead. For example, like in Pascal (var) or C++ (& in function declaration).

[shkkmo → MJSplot_author]

Yes, which together with the other example shows that javascript uses pass-reference-by-value, as explained elsewhere on the thread.