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Disallow -0 in algorithm for checking array index #517

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Feb 9, 2018
Merged

Disallow -0 in algorithm for checking array index #517

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c8a8ec7
Disallow -0 in algorithm for checking array index
TimothyGu Jan 31, 2018
f7df2c8
fixup!
tobie Feb 7, 2018
d2ff86d
fixup!
tobie Feb 7, 2018
b3a2201
fixup! address @TimothyGu's comments.
tobie Feb 7, 2018
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5 changes: 5 additions & 0 deletions index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -181,6 +181,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
text: error objects; for: ECMAScript; url: sec-error-objects
text: function object; url: function-object
text: immutable prototype exotic object; url: sec-immutable-prototype-exotic-objects
text: integer index; url: integer-index
url: sec-object-internal-methods-and-internal-slots
text: internal method
text: internal slot
Expand Down Expand Up @@ -12625,6 +12626,10 @@ internal method as follows.
applied:

1. If <a abstract-op>Type</a>(|P|) is not String, then return <emu-val>false</emu-val>.
1. If |P| is "<code>-0</code>", then return <emu-val>false</emu-val>.

Note: <a abstract-op>CanonicalNumericIndexString</a> allows "<code>-0</code>" as a special
case, even though it is not an [=integer index=] or by extension an [=array index=].
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If this indeed matches implementations, wouldn't it be cleaner to return false if index is -0 after the following step?

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The implementation for this in Chromium:
https://chromium.googlesource.com/v8/v8.git/+/842d424d99060375bf4a8b6fdfd2a0177e4c0477/src/api.cc#4128
https://chromium.googlesource.com/v8/v8.git/+/842d424d99060375bf4a8b6fdfd2a0177e4c0477/src/utils-inl.h#35

So no special handling of "-0" there because the code uses unsigned integers. ES uses ToNumber where negative zero is possible, so whatever this says won't be a close match.

1. Let |index| be [=!=] <a abstract-op>CanonicalNumericIndexString</a>(|P|).
1. If |index| is <emu-val>undefined</emu-val>, then return <emu-val>false</emu-val>.
1. If |index| is less than 0 or is greater than or equal to 2<sup>32</sup> − 1, then return
Expand Down