Javascript shallow copies

I'm fairly new to Javascript and ran into a problem with object references. Here's a snippet of the code that reproduces the issue.

function MyVector() {
    this.x = 0;
    this.y = 0;
}

function MySprite() {
    this.pos = new MyVector();
}

var my_sprite = new MySprite(); 
var my_pos = my_sprite.pos;    // my_pos points to my_sprite's vector
var my_vect = new MyVector();  // my_vector is on the heap
my_vect.x = 69;

print("my_pos.x: " + my_pos.x);  // prints 0 (expected)

my_sprite.pos = my_vect;         // should point to my_vect

print("my_sprite.pos.x: " + my_sprite.pos.x); // prints 69 (expected)
print("my_pos.x: " + my_pos.x);  // prints 0, expected to print 69

In the last print statement, I would've expected my_pos.x to print 69 since my_pos now refers to my_vect (or does it?). My understanding of Javascript is that only primitive values are actually copied and that object assignment maintains a reference to a single copy of the object. It looks like after the assignment of my_vect, the my_pos reference is somehow "detached" and no longer points to my_sprite.pos.

Is there some behavior of Javascript that I'm overlooking here?

开发者_如何转开发

Thanks!

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Your code with more comments, hope this helps

function MyVector() {
    this.x = 0;
    this.y = 0;
}

function MySprite() {
    this.pos = new MyVector();
}

var my_sprite = new MySprite();

at this point, a new MyVector() is created and assigned to my_sprite.pos. Let call it vector#1

var my_pos = my_sprite.pos;    // my_pos points to my_sprite's vector

my_pos points to vector#1 (you assign the pointer, not the data)

var my_vect = new MyVector();  // my_vector is on the heap

a new MyVector is created let's call it vector#2. Also, my_vect points to vector#2

my_vect.x = 69;

print("my_pos.x: " + my_pos.x);  // prints 0 (expected)

my_sprite.pos = my_vect;         // should point to my_vect

the pos member of my_sprite now points to vector#2

print("my_sprite.pos.x: " + my_sprite.pos.x); // prints 69 (expected)

but my_pos still points to vector#1 as it was never re-assigned!

print("my_pos.x: " + my_pos.x);  // prints 0, expected to print 69

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Your not overlooking anything.

var my_sprite = new MySprite(); 

so here,

my_sprite.pos == new myVector(); 
var my_pos = my_sprite.pos;  // my_pos equals that new myVector.

You never re-assign my_pos, so it still equals that new myVector when you print it. When assigning variables in javascript, my_pos does not point to my_sprite.pos since my_sprite.pos is also a pointer, so it simple points to what my_sprite.pos points to.