I was helping a colleague debug some code today and I noticed a strange behavior with console.log() in Google Chrome:
It appears that if you:
Create a nested array (e.g., [[345,"test"]])
Log the array to the console with console.log().
Modify one of the inner array values, then console.log() will output the later value -- not the values of the array at the time the console.log() was executed.
JavaScript:
var test = [[2345235345,"test"]]
console.log(test);
test[0][0] = 1111111;
// outputs: [[1111111,"test"]]
var testb = {};
testb.test = "test";
console.log(testb);
testb.test = "sdfgsdfg";
// outputs: {"testb":"test"}
var testc = ["test","test2"];
console.log(testc);
testc[0] = "sdxfsdf";
// outputs: ["test","test2"]
JSFiddle Example
This behavior does not happen in Firefox.
Also to note, if I stepped through his code line by line in the Chrome debugger, then console.log() would output the correct values.
Is there an explanation for this strange phenomenon or is it just a bug with Google Chrome?
EDIT:
I've narrowed down the steps to reproduce the inconsistent console.log() behavior:
If you add this script to your page:
var greetings=['hi','bye'];
console.log(greetings);
setTimeout(function(){
greetings.push('goodbye');
},3000);
and open it in a new window with the Chrome console window already open, then the console.log() output will be different than if you load the page with the console window closed. Here's a JSFiddle that demonstrates that.
In the first case, with the console window already open, console.log() will output the current value of the array (i.e., two items).
In the second case, with the console window initially closed and opened only after the page loads, console.log() will output the later values of the array (i.e., three items).
Is this a bug in Google Chrome's console.log() functionality?
After a lot of digging, I found that this has been reported as a bug, fixed in Webkit, but apparently not yet pulled into Google Chrome.
As far as I can tell, the issue was originally reported here: 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.
A fix was implemented two and a half years later on August 9th, 2012 for Webkit ( http://trac.webkit.org/changeset/125174 ), but it does not appear to have made it into Chrome yet.
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.
I found a workaround for this bug/feature.
console.log(JSON.parse(JSON.stringify(myObject)));
Edit: Unfortunately this won't work for non-primitive values like functions. Use another clone utility here.
jQuery example:
console.log($.extend({}, myObject));
Related
This question already has answers here:
Weird behavior with objects & console.log [duplicate]
(2 answers)
Closed 2 years ago.
If i run the following code on chrome or firefox directly into the console, on the first log the value of value2 is 0 in the inlined representation, but if I expand the representation to check the whole object, it is showed as 17. Is this a multi browser bug or I am missing something with javascript here? The reason I am asking this question here is because this behaviour is happening across different browsers, which makes me think there is some kind of catch on the language or the function that I am not aware of.
Actual code:
action = new Object();
action.value1 = 17;
action.value2 = 0;
console.log(action);
action.value2 = action.value1;
console.log(action);
Tested on MacOS Catalina with the latest version of both browsers.
EDIT-----------------
The problem is the first console.log prints value2 being 0 but when I expand the object details it is showed as 17. I attached a print to clarify the question.
PRINT OF THE CONSOLE RESULTS
I would expect the result to be 0 on the value2 of the first output even with the expanded view.
console.log has different inner working between chrome, firefox, nodejs, and other javascript engine.
In your screenshot, the browser does 2 things:
prints the short preview of the action object
prints the expandable reference of the action object
The short preview is a readable version snapshot of the object at the time console.log is called
The expendable reference only prints an expand icon at the time console.log is called.
When expanded, chrome will fetch the CURRENT value the object referenced by that expendable reference and print it. It doesn't print the value when console.log is called
Since the value when the console.log is called ({value1: 17, value2: 0}), and the current value ({value1: 17, value2: 17}) is different, it prints different thing.
Why the result of printing x using console.log is undefined?
Update:
This is not a duplicate. I'm trying to understand why console.log doesn't print.
I'm not talking about the return value of console.log.
So I dig a little bit and I found something that could of happen to you.
when you open your Dev Tools, you have this little icon on the left:
if you clicked on it, it will open a sidebar and if you are marking one of the two options that I point, it will not show the console.log print.
Change to the first selection, and you'll find out that you get the print.
Logging to the console is a redundant action in devtools, since it is bound to output to the console any values that are passed to it.
x
Is the proper way to print the value of x to the console.
console.log(x)
Is basically the same as writing
console.log(console.log(x))
Which returns undefined. Though, if you have an active debugger, it will evaluate and also print the value
I found an answer here: Chrome: console.log, console.debug are not working:
By #Tim:
Same issue, but I just cleared my settings. I went into Settings > Preferences and Clicked [Restore defaults and reload]. Just remember what your settings were.
I was helping a colleague debug some code today and I noticed a strange behavior with console.log() in Google Chrome:
It appears that if you:
Create a nested array (e.g., [[345,"test"]])
Log the array to the console with console.log().
Modify one of the inner array values, then console.log() will output the later value -- not the values of the array at the time the console.log() was executed.
JavaScript:
var test = [[2345235345,"test"]]
console.log(test);
test[0][0] = 1111111;
// outputs: [[1111111,"test"]]
var testb = {};
testb.test = "test";
console.log(testb);
testb.test = "sdfgsdfg";
// outputs: {"testb":"test"}
var testc = ["test","test2"];
console.log(testc);
testc[0] = "sdxfsdf";
// outputs: ["test","test2"]
JSFiddle Example
This behavior does not happen in Firefox.
Also to note, if I stepped through his code line by line in the Chrome debugger, then console.log() would output the correct values.
Is there an explanation for this strange phenomenon or is it just a bug with Google Chrome?
EDIT:
I've narrowed down the steps to reproduce the inconsistent console.log() behavior:
If you add this script to your page:
var greetings=['hi','bye'];
console.log(greetings);
setTimeout(function(){
greetings.push('goodbye');
},3000);
and open it in a new window with the Chrome console window already open, then the console.log() output will be different than if you load the page with the console window closed. Here's a JSFiddle that demonstrates that.
In the first case, with the console window already open, console.log() will output the current value of the array (i.e., two items).
In the second case, with the console window initially closed and opened only after the page loads, console.log() will output the later values of the array (i.e., three items).
Is this a bug in Google Chrome's console.log() functionality?
After a lot of digging, I found that this has been reported as a bug, fixed in Webkit, but apparently not yet pulled into Google Chrome.
As far as I can tell, the issue was originally reported here: 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.
A fix was implemented two and a half years later on August 9th, 2012 for Webkit ( http://trac.webkit.org/changeset/125174 ), but it does not appear to have made it into Chrome yet.
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.
I found a workaround for this bug/feature.
console.log(JSON.parse(JSON.stringify(myObject)));
Edit: Unfortunately this won't work for non-primitive values like functions. Use another clone utility here.
jQuery example:
console.log($.extend({}, myObject));
I think the pictures are relatively self explanatory (code matched with console output)
Variable appears to be set to a value, except immediate console.log afterwards outputs something different.
Code:
Console Output
Extended console output (as requested in comments) (extra console.log before and after)
That is not actually a bug.
When you print a primitive, you'll see the state of it when you printed it.
When you work with objects, if you console.log them, it only shows the reference. When you click on them, it will show you their current state. So what happened in your case is that the answer probably got changed in a later code.
I was helping a colleague debug some code today and I noticed a strange behavior with console.log() in Google Chrome:
It appears that if you:
Create a nested array (e.g., [[345,"test"]])
Log the array to the console with console.log().
Modify one of the inner array values, then console.log() will output the later value -- not the values of the array at the time the console.log() was executed.
JavaScript:
var test = [[2345235345,"test"]]
console.log(test);
test[0][0] = 1111111;
// outputs: [[1111111,"test"]]
var testb = {};
testb.test = "test";
console.log(testb);
testb.test = "sdfgsdfg";
// outputs: {"testb":"test"}
var testc = ["test","test2"];
console.log(testc);
testc[0] = "sdxfsdf";
// outputs: ["test","test2"]
JSFiddle Example
This behavior does not happen in Firefox.
Also to note, if I stepped through his code line by line in the Chrome debugger, then console.log() would output the correct values.
Is there an explanation for this strange phenomenon or is it just a bug with Google Chrome?
EDIT:
I've narrowed down the steps to reproduce the inconsistent console.log() behavior:
If you add this script to your page:
var greetings=['hi','bye'];
console.log(greetings);
setTimeout(function(){
greetings.push('goodbye');
},3000);
and open it in a new window with the Chrome console window already open, then the console.log() output will be different than if you load the page with the console window closed. Here's a JSFiddle that demonstrates that.
In the first case, with the console window already open, console.log() will output the current value of the array (i.e., two items).
In the second case, with the console window initially closed and opened only after the page loads, console.log() will output the later values of the array (i.e., three items).
Is this a bug in Google Chrome's console.log() functionality?
After a lot of digging, I found that this has been reported as a bug, fixed in Webkit, but apparently not yet pulled into Google Chrome.
As far as I can tell, the issue was originally reported here: 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.
A fix was implemented two and a half years later on August 9th, 2012 for Webkit ( http://trac.webkit.org/changeset/125174 ), but it does not appear to have made it into Chrome yet.
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.
I found a workaround for this bug/feature.
console.log(JSON.parse(JSON.stringify(myObject)));
Edit: Unfortunately this won't work for non-primitive values like functions. Use another clone utility here.
jQuery example:
console.log($.extend({}, myObject));