How does reassigning the value of an objects attribute work in js

Asked Jul 23 '20 at 13:01

Active Jul 23 '20 at 13:04

Viewed 33 times

What is going on in the following js code?

let testObj={x:[1,2,3],y:[1,2,3]};
console.log(testObj);
testObj.x=[1,2,2]
testObj.y=[1,1,1]
console.log(testObj);

Output

{x: Array(3), y: Array(3)}
x: (3) [1, 2, 2]
y: (3) [1, 1, 1]
__proto__: Object

{x: Array(3), y: Array(3)}
x: (3) [1, 2, 2]
y: (3) [1, 1, 1]
__proto__: Object

Surely the first console.log should have run before I re-assigned the attributes of testObj? What is happening?

asked Jul 23 '20 at 13:01

Vishal Jain

Is this in chrome developer tools? – Nick Parsons Jul 23 '20 at 13:02
Yes this is from chromes console – Vishal Jain Jul 23 '20 at 13:02
see this answer: [Is Chrome's JavaScript console lazy about evaluating arrays?](https://stackoverflow.com/q/4057440) (same idea applies to objects) – Nick Parsons Jul 23 '20 at 13:02
So a bug from 10 years ago still hasnt been fixed? At least I know its a bug I guess – Vishal Jain Jul 23 '20 at 13:04
Or this: https://stackoverflow.com/questions/24175017/google-chrome-console-log-inconsistency-with-objects-and-arrays – MauriceNino Jul 23 '20 at 13:04
4

It's not a bug, it's working exactly as intended. If you want to freeze the object in time, you'll need to deep-copy it, or export it to JSON. – Niet the Dark Absol Jul 23 '20 at 13:05
Also, you should see an `[i]` (information icon) which you can however over which says "Value below was evaluated just now", which allows you to see if you're seeing the "live" version or a "snapshot" – Nick Parsons Jul 23 '20 at 13:12
I see, makes more sense now thanks – Vishal Jain Jul 23 '20 at 13:51

How does reassigning the value of an objects attribute work in js

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