javascript - JS object copy by value vs copy by reference

Question

I was playing with chrome console and noticed something which I couldn't understand. I know in JS variables are copied by value and objects are copied by reference. Below code works fine as expected which outputs 2 and proves JS objects work as reference:

var objA = {a: 1};
var objB = objA;
objA.a = 2; 
objB.a; // 2

However this code doesn't work as it should. I expected objB.a to output 2 but it gives 1 instead. Why?

var objA = {a: 1};
var objB = objA;
objA = {a: 2};  //Assigned whole object here instead property.
objB.a; //1 - Shouldn't this be 2 ??

Answer 1

I'd rather think of variables with objects as pointers to objects (like C pointers) rather than references.

In your third line, you just replaced objA, making it "point to" another object. It does not change whatever objB is "pointing".

By line 3, objA now points to {a:2} while objB still points to whatever objA was pointing at the time you assigned it to objB, at line 2, which is {a:1}.

line 1: objA -> {a:1}
line 2: objA -> {a:1} <- objB
line 3: objA -> {a:2}, objB -> {a:1}