ties are rounded differently on c and js backends

[skilchen]

On the c-like backends ties (values exactly between 2 adjacent integers) are rounded "away from zero".
On the js backend ties are rounded toward positive infinity.

import math
echo round(-1.5)

produces -2 on the c backend, but -1 on the js backend.

[skellock]

That's interesting that the JS spec calls that out directly:

If the fractional portion is exactly 0.5, the argument is rounded to the next integer in the direction of +∞. Note that this differs from many languages' round() functions, which often round this case to the next integer away from zero, instead giving a different result in the case of negative numbers with a fractional part of exactly 0.5.

Emphasis is theirs.

[skilchen]

Another rounding differenc:
In the stringification all backends round ties "away from zero", but
on the c backends Nim rounds ties to nearest even
on the js backend Nim rounds ties up or down according to the sign of the float.

import strutils
echo formatFloat(2.5, ffDecimal, 0))

produces 2. on the c backend, but 3 on the js backend.
On the c backends we also have the enforced decimal point, which is missing on the js backend.

The round procedure never rounds ties to nearest even: echo round(2.5) produces 3.0 on all backends.