I was reading some parsing/lexing code when I thought to question how
named literals were being handled - 'true', 'false', and 'null'. So
I went back to the 3.1 spec and then tested with four well-known ES
implementations, FF3, Opera9, Chrome, and IE7. If anything, I'm less
confused but now more worried at the difference between practice and
(apparent) definition of how to handle these literals.
For reference, some parts of the 3.1 Annex A Grammar:
Identifier :: See 7.6
IdentifierName but not ReservedWord
ReservedWord :: See 7.5.1
Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
BooleanLiteral :: See 7.8.2
true
false
PropertyName : See 11.1.5
IdentifierName
StringLiteral
NumericLiteral
PrimaryExpression : See 11.1
...
Identifier
MemberExpression : See 11.2
...
MemberExpression . IdentifierName
And some testing with the four current implementations of ES:
FF b=new Object(); b.true = false; b.true; --> 'false'
Chrome b=new Object(); b.true = false; b.true;
SyntaxError: Unexpected token true
Opera >>> b=new Object(); b.true = false; b.true;
Asynchronous script compilation
Syntax error : line 1 of unknown script :
Expected identifier
b=new Object(); b.true = false; b.true;
----------------------^
IE7 b=new Object(); b.true = false; b.true;
1234567890123456789
Line: 1
Char: 19
Error: Expected identifier
FF a = { true: false, false: true }; a.true; --> 'false'
Chrome a = { true: false, false: true }; a.true;
SyntaxError: Unexpected token true
Opera >>> a = { true: false, false: true }; a.true;
Asynchronous script compilation
Syntax error : line 1 of unknown script :
Expected token: '}'
a = { true: false, false: true }; a.true;
----------^
IE7 a = { true: false, false: true }; a.true;
Line: 1
Char: 7
Error: Expected identifier, string or number
(for completeness, note similar results are obtained for
a = { if: false, then: true }; a.if;
FF is agreeable, the others are not)
FF a = { true: false, false: true }; with (a) true; --> 'true'
a = { truth: false, lies: true }; with (a) truth; --> 'false'
(the others reject keyword 'true' as already shown)
The ES 3.1 draft spec appears to say that 'true' and the others are
valid 'Identifier's for use in at least the two places tested above,
PropertyName and MemberExpression.
Testing shows that only FireFox allows this, or at least does in two
of three tests above. But given the failure under the 'with'
construction, I wonder how well it understands the literal
truth. (Or is that explained by rule "PrimaryExpression : Identifier" ?)
So....
Was it intended that the named literals 'true', 'false', and 'null'
(and the keywords) were to be included as valid Identifier's?
How disturbing is it that most of the implementations disagree with
this definition?
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