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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: x === x
Date: Wed, 31 May 2017 23:39:41 +0200
Organization: PointedEars Software (PES)
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Message-ID: <2276977.Lt9SDvczpP@PointedEars.de>
References: <JavaScript-20170530123255@ram.dialup.fu-berlin.de> <874lw231ui.fsf@bsb.me.uk> <context-20170530144535@ram.dialup.fu-berlin.de> <2872662.aeNJFYEL58@PointedEars.de> <87inkh28sf.fsf@bsb.me.uk> <19437984.EfDdHjke4D@PointedEars.de> <87efv4zr3k.fsf@bsb.me.uk>
Reply-To: Thomas 'PointedEars' Lahn <cljs@PointedEars.de>
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Ben Bacarisse wrote:

> Thomas 'PointedEars' Lahn <PointedEars@web.de> writes:
>> | > {} === {}
>> | <- false
>> | {} === {};
>> | (x) Uncaught SyntaxError: Unexpected token ===
>>
>> I assume that the explicit semicolon there is considered an
>> /EmptyStatement/ that switches back to statement context.
> 
> I can't find any reference to "statement context" or "expression
> context" in the language standard.  I assume it's your own terminology,
> but I don't see how it helps explain why [] === []; is OK but {} === {};
> is a syntax error.  (I know why, I'm just wondering how talking about
> === making expression context and ; switching back helps to explain this
> difference.)

Simply put, ECMAScript syntax is ambiguous, and it depends on the context 
how it is parsed.  For example, “{}” can be parsed either as an empty 
/Block/ *statement* (which can only be produced by the /Statement/ 
production) or as an empty Object initializer; the latter is an *expression* 
(it can only be produced by the /Expression/ production).  A further 
complication is the /ExpressionStatement/ production (IIRC).
 
I will consider the rest of your posting later.

-- 
PointedEars
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