Floating point value decoded incorrectly by buf.readDoubleBE()

[DavidRusso]

• Version: v9.4.0
• Platform: Windows 10 Pro x64 Version 1709 (OS Build 16299.192)
• Subsystem: Buffer

example_js.txt

The attached example produces the output:

246800 encodes to 410e208000000000
410e208000000000 decodes to 246800.03176522627

The expected result is:

246800 encodes to 410e208000000000
410e208000000000 decodes to 246800

[apapirovski]

/cc @bnoordhuis this is likely related to the recent switch to calculating this in JS.

[mrmacete]

Here's another example:

v9.4.0 (wrong):

> const x = Buffer.alloc(8,0)
undefined
> x.writeDoubleLE(41.0)
8
> x.toString('hex')
'0000000000804440'
> Buffer.from('0000000000804440','hex').readDoubleLE()
0

v8.9.4 (correct):

> const x = Buffer.alloc(8,0)
undefined
> x.writeDoubleLE(41.0)
8
> x.toString('hex')
'0000000000804440'
> Buffer.from('0000000000804440','hex').readDoubleLE()
41

edit: ok this seems to happen just because the offset argument here is undefined and in previous versions it was coerced to 0, so passing in an explicit 0 will do.

[mrmacete]

also, probably the confusion comes from the fact that the examples in the docs actually pass undefined there (even if offset isn't stated as optional): https://nodejs.org/api/buffer.html#buffer_buf_readdoublele_offset_noassert