Variable is different when using it directly

Question

The Javascript engine of Google Chrome does something strange with the variable values in this prototype.

The prototype is defined with:

function matrix4def()
{
 this.m00=0;
 this.m01=0;
 this.m02=0;
 this.m03=0;
 this.m10=0;
 this.m11=0;
 this.m12=0;
 this.m13=0;
 this.m20=0;
 this.m21=0;
 this.m22=0;
 this.m23=0;
 this.m30=0;
 this.m31=0;
 this.m32=0;
 this.m33=0;
};

var value = new matrix4def();

console.log(value) shows us:

 m00: 1
 m01: 0
 m02: 0
 m03: 0
 m10: 0
 m11: 1
 m12: 0
 m13: 0
 m20: 0
 m21: 0
 m22: 1
 m23: 0
 m30: 0
 m31: 0
 m32: 0
 m33: 1

While using console.log(value.m00) will show us: -6.123031769111886e-17. Doing console.log(value); console.log(value.m00); console.log(value); shows no difference between the first and second call of console.log(value);.

Doing console.log(typeof(value.m00)) will show us: number

I've tried doing parseFloat(value.m00), but that didn't show 1 either... Storing the value of value.m00 to a temporary value shows me -6.123031769111886e-17 too.

Does anyone have a clue what's happening here?

P.S.: I cannot show you the entire code; it's part of render code that does a lot with these values.

I don't see this happending. How are you creating your object "value"? — Ashwin Prabhu, Sep 17 '12 at 11:02
`var value = new matrix4def();` I must say that it works on all internal calculations, but when you try to see the actual result, you get wrong info. — Diamondo25, Sep 17 '12 at 11:04
Floats and integers are both of the type `number` in JavaScript, just FYI. — J. K., Sep 17 '12 at 11:08
This code does indeed work, but there's a problem with the 'transfer to text' code of either the Javascript engine. I wonder why that happens. — Diamondo25, Sep 17 '12 at 11:10
@Bergi It would be too complicated to change this code to a twodimensional array, as the variables are used almost everywhere. — Diamondo25, Sep 17 '12 at 11:11
@Diamondo25: It will just become a pain if you need to change it to `matrix5def` or the like… — Bergi, Sep 17 '12 at 11:17
What input goes to the `parseFloat` call? I don't really see the need to do text-to-number conversion. — J. K., Sep 17 '12 at 11:17
@IanKuca typeof(value.m00) shows me that it's a number, yes. — Diamondo25, Sep 17 '12 at 11:36
Then any conversion (parseFloat) is useless because `parseFloat(value) === value` in your case. I don't really see your issue. — J. K., Sep 17 '12 at 11:39
@IanKuca It was a try to force it to be a correct value, as I thought it might've been a different type of value. — Diamondo25, Sep 17 '12 at 11:44

score 2 · Answer 1 · answered Sep 17 '12 at 11:11

2

This looks like an issue with console.log, which show you the current property values of an object instead of the ones it had when the object was logged. Do you have any code around (after) the logging that changes the values?

console.log(new matrix4def().m00)

will log 0, not something negative very close to zero.

answered Sep 17 '12 at 11:11

Bergi

630,263
148
957
1,375

I'm agree with you, becourse in my demo, it's running well, and dosen't have some strange result – xiaohao Sep 17 '12 at 11:12
The `console.log` is ran with the final values of `value`, that will be used to render the objects in WebGL. – Diamondo25 Sep 17 '12 at 11:13
Does the WebGL code change the values? Show it to us, create a http://jsfiddle.net for it. – Bergi Sep 17 '12 at 11:17
@Bergi Running `console.log(value); console.log(value.m00); console.log(value);` shows me that, internally, `value.m00` is not changed at all, but `console.log(value.m00)` still shows me the `-6.123...` – Diamondo25 Sep 17 '12 at 11:22

score 0 · Answer 2 · edited May 23 '17 at 10:25

0

I suspect you encounter this issue when running your full script (or at least: after performing some operations on the properties of your object). And, given that you're using parseFloat, I'm assuming the values can be decimal (float) values.
If that's the case, you should take a closer look at the known issues with JS and floating point numbers:

either here
or here
maybe here, too
and last but not least: here

Don't forget to look into the articles that are referred to on the last page

edited May 23 '17 at 10:25

Community

1
1

answered Sep 17 '12 at 11:19

Elias Van Ootegem

74,482
9
111
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I've tried using `console.log(value.m00.toFixed(5));`, but it gives me `-0.00000` O.o. Using `console.log(value.m00.toPrecision(5));` gives `-6.1230e-17` – Diamondo25 Sep 17 '12 at 11:35
`toFixed` has [issues, too](http://stackoverflow.com/questions/661562/how-to-format-a-float-in-javascript/661757#661757). If needs must, you can use Christophe's generic `toFixed` function, and attach that to the `Number.prototype`. That should work – Elias Van Ootegem Sep 17 '12 at 11:47
The `toFixed` function returns '0' :( – Diamondo25 Sep 17 '12 at 12:37
Nope, it should've returned '1' – Diamondo25 Sep 17 '12 at 13:03
Weird, `parseFloat(value.m00.toFixed(5)).toPrecision(5);`? _desparate, I know_, but returns 0.00000 here – Elias Van Ootegem Sep 17 '12 at 13:22

Variable is different when using it directly

2 Answers2