Friday, May 10, 2024
177
rated 0 times [  181] [ 4]  / answers: 1 / hits: 63362  / 12 Years ago, sun, july 1, 2012, 12:00:00

This bit of code I understand. We make a copy of A and call it C. When A is changed C stays the same


var A = 1;
var C = A;
console.log(C); // 1
A++;
console.log(C); // 1

But when A is an array we have a different situation. Not only will C change, but it changes before we even touch A


var A = [2, 1];
var C = A;
console.log(C); // [1, 2]
A.sort();
console.log(C); // [1, 2]

Can someone explain what happened in the second example?


More From » google-chrome

 Answers
32

Pointy's answer has good information, but it's not the correct answer for this question.



The behavior described by the OP is part of a bug that was first reported in March 2010, patched for Webkit in August 2012, but as of this writing is not yet integrated into Google Chrome. The behavior hinges upon whether or not the console debug window is open or closed at the time the object literal is passed to console.log().



Excerpts from the original bug report (https://bugs.webkit.org/show_bug.cgi?id=35801):




Description From mitch kramer 2010-03-05 11:37:45 PST



1) create an object literal with one or more properties



2) console.log that object but leave it closed (don't expand it in the console)



3) change one of the properties to a new value



now open that console.log and you'll see it has the new value for some reason, even though it's value was different at the time it was generated.



I should point out that if you open it, it will retain the correct value if that wasn't clear.




Response from a Chromium developer:




Comment #2 From Pavel Feldman 2010-03-09 06:33:36 PST



I don't think we are ever going to fix this one. We can't clone object upon dumping it into the console and we also can't listen to the object properties' changes in order to make it always actual.



We should make sure existing behavior is expected though.




Much complaining ensued and eventually it led to a bug fix.



Changelog notes from the patch implemented in August 2012 (http://trac.webkit.org/changeset/125174):




As of today, dumping an object (array) into console will result in objects' properties being
read upon console object expansion (i.e. lazily). This means that dumping the same object while
mutating it will be hard to debug using the console.



This change starts generating abbreviated previews for objects / arrays at the moment of their
logging and passes this information along into the front-end. This only happens when the front-end
is already opened, it only works for console.log(), not live console interaction.



[#84541] Friday, June 29, 2012, 12 Years  [reply] [flag answer]
Only authorized users can answer the question. Please sign in first, or register a free account.
mireyag

Total Points: 73
Total Questions: 107
Total Answers: 85

Location: Ukraine
Member since Sun, Dec 13, 2020
3 Years ago
mireyag questions
Sun, Aug 15, 21, 00:00, 3 Years ago
Wed, Dec 16, 20, 00:00, 3 Years ago
Tue, Sep 1, 20, 00:00, 4 Years ago
Sun, Jul 5, 20, 00:00, 4 Years ago
;