251855 - .toFixed() seems to have a problem with math precision (returning more digits than wanted)

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[Philip Timmermann]

User-Agent:       Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.7) Gecko/20040628 Firefox/0.9.1
Build Identifier: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.7) Gecko/20040628 Firefox/0.9.1

I just did a very simple thing:

   for (i=0; i<11; i++)
   {
      alert( i + " = " + ( i * ( 100 / 255 ).toFixed(2) ) );
   }

...and got the following result:

   0 = 0
   1 = 0.39
   2 = 0.78
   3 = 1.17
   4 = 1.56
   5 = 1.9500000000000002
   6 = 2.34
   7 = 2.73
   8 = 3.12
   9 = 3.5100000000000002
   10 = 3.9000000000000004

I'm surprised that the .toFixed(2) thing isn't just able to omit the last digits
in EVERY case and give me back the value it's supposed to. It should be more
powerful than what I get now. If I state to want back a .toFixed(2) number
SpiderMonkey should be able to return it to me in any case, right?


Reproducible: Always
Steps to Reproduce:
1.
2.
3.

Comment 1

[Erik Fabert]

The problem here is not with toFixed. You are multiplying the value returned by 
toFixed again, this is adding the digits:

( 100 / 255 ).toFixed(2) => 0.39
10 * 0.39 = > 3.9000000000000004

Btw., IE6 returns 3.9000000000000003. Don't know which result is correct, I 
guess Mozilla's.

Comment 2

[Philip Timmermann]

I really should have seen this myself... :-(

Sorry for wasting your time...

This bug is invalid...