Pass-by-reference JavaScript objects

Question

Please read the comments in the code below to know what I am trying to ask.

Expected Output: As per pass-by-reference mechanism in JavaScript, objOne is expected to log {} at the end, because objTwo was initialized with {}.

var objOne = {   x: 1,   y: 2 };  var objTwo = objOne;  // change the x vlaue to 2 by objTwo objTwo.x = 2;  // Change the value of key x in objOne as well - pass by reference mechanism console.log(objOne); // { x: 2, y: 2 }  /*** Pass by reference is understood in code, above this comment ***/  // Now what if objTwo initialized with empty object objTwo = {};  console.log(objOne); // { x: 2, y: 2 } but expected output = {}  // As per pass by reference mechanism. objOne is expected to log {}, because objTwo was initialized with {}. 

Answer 1

When you assign one variable to another, it's not that both those variables are now linked by reference; you're misunderstanding what "pass by reference" means here.

A variable holding an object does not "directly" hold an object. What it holds is a reference to an object. When you assign that reference from one variable to another, you're making a copy of that reference. Now both variables hold a reference to an object. Modifying the object through that reference changes it for both variables holding a reference to that object.

When you assign a new value to one of the variables, you're just modifying the value that variable holds. The variable now ceases to hold a reference to the object, and instead holds something else. The other variable still holds its reference to the original object, the assignment didn't influence it at all.

Answer 2

When you evaluate

objTwo = {}; 

Javascript interprets that as reassigning objTwo to a new literal empty object, and leaves its old value alone.

If you want to remove a key from objOne by reference, you can use the delete keyword:

delete objTwo.x;  // also deletes the x property of objOne