fix printing negative zero in JS backend

[ringabout]

see timotheecour#136

before this doesn't work in JS backend(echo $a prints 0.0 instead of -0.0).

proc main()=
  block:
    let a = -0.0
    doAssert $a == "-0.0"
    doAssert $(-0.0) == "-0.0"


static: main()
main()

• VM prints 0.0 as -0.0, I will fix it in the following PR.

[ringabout]

see also scala.js
https://github.com/scala-js/scala-js/blob/9857514d6b7482d3b62ff37e254dc2118ff51f25/javalib/src/main/scala/java/util/Formatter.scala#L509

scala-js/scala-js#1011

[ringabout]

Unfortunately, the VM bug is not easy to solve

Uncomment the codes cause the memory of y is polluted

proc main()=
  # block:
  #   let x = -0.0
  #   doAssert $x == "-0.0"

  block:
    let y = 0.0
    doAssert $y == "0.0"


static: main()

What's your opinion? @timotheecour

[timotheecour]

@xflywind I just found out about Object.is, we should use it instead, it seems super useful here as well as in your copySign PR

if(Object.is(`a`, -0)) `result` = "-0.0"
else if(Number.isSafeInteger(`a`)) `result` =  `a`+".0"
else { ...

ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is

we can add the polyfill provided there for IE, if needed

(it may have other uses too, see examples: Object.is([], []); // false) which are hard to do without Object.is

[ringabout]

Is Object.is better than a === 0 && 1 / a < 0?

we can add the polyfill provided there for IE, if needed

It emits more codes if we need to support IE.

I will think of it and look at whether scala.js uses Object.is.

[timotheecour]

Should be more efficient; the more code emitted doesn't matter for platforms other than IE as it won't be used

[timotheecour]

@xflywind

VM prints 0.0 as -0.0, I will fix it in the following PR.
Unfortunately, the VM bug is not easy to solve
Uncomment the codes cause the memory of y is polluted

are you using an old nim? I've already fixed this in #16470; it works fine in latest nim devel 95f599c

This test needs to be updated:

when nimvm: discard ## TODO VM print wrong -0.0

since it works