JS Daily 26 - Object Copy By Reference

Last updated on 16 Dec, 2020

What is the output of following code?

const obj1 = {
  a : "Apple",
  showA : function() {

const obj2 = obj1;

obj2.a = "Banana";


The output is "Banana".

In line1, an object is created and assigned to variable obj1. Note that, I did not say "we created an object obj1". It is because, ideally what is happening is that, at first, JavaScript engine creates the object on the right hand side somewhere in the memory. It then stores the reference of that object in variable obj1.

Next in line 8, when we copy obj1 to obj2, only the reference to the object is copied. That is why, in line 10, when we change the value of obj2.a, it is reflected in the original object.

Finally, in line 12, even though we are calling method showA() of obj1, the changed value of a ("Banana") is printed.

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Joby Joseph
Web Architect