Javascript pointer/reference craziness. Can someone explain this?
Javascript passes objects by reference. This makes perfect sense. But once you start manipulating those objects, everything acts in a way that seem unintuitive. Let me offer an example:
var a, b;
a = {}
b = a;
a['one'] = {};
console.log( JSON.stringify(a) );
// outputs: {"one":{}}
console.log( JSON.stringify(b) );
// outputs: {"one":{}}
This is all well and good because now b has a pointer to a so it's expected that assigning stuff to a will also affect b.
But then if I do this:
a = a['one'];
console.log( JSON.stringify(a) );
// outputs: {}
console.log( JSON.stringify(b) );
// outputs: {"one":{}}
This is surprising to me. I'd expect a and b to still be the same (and to be {} since a['one'] was previously set to {} and a was set to a['one']).
But that's not the case. It appears that a loses its reference to b when it's assigned to something new, but b maintains the value that a was set to prior to a loosing its reference to b.
But then if I do this:
a['two'] = 2;
console.log( JSON.stringify(a) );
// outputs: {"two":2}
console.log( JSON.stringify(b) );
// outputs: {"one":{"two":2}}
What? a has clearly lost it's reference to b, but b seems to still have some reference to a.
Does the empty object {} point to some place in memory so every variable referencing it is now pointing to the same place?
Can someone with a firm grasp on this explain it to me?
Answer
Following your example line by line:
a = {}
a now references the new object.
b = a;
b now references the same object that a references. Note that it does not reference a.
a['one'] = {};
The new object now has an index 'one' that references another new object.
When you do
a = a['one'];
You are setting a to refer to a['one'], which is that new object you created when you did a['one'] = {}. b still references the object you created with a = {}.
You are confusing the issue when you say "a has lost its reference to b" because a does not refer to b , nor vice versa. a and b refer to objects, and they can be made to refer to other objects. Like this:
With a = {}; b = a, you get
a
\
\
{ }
/
/
b
Then with a['one'] = {} you get
a
\
\
{ one: { } }
/
/
b
Then with a = a['one'] you get
a - - - -
\
{ one: { } }
/
/
b