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ExpressionNoIn in Javascript and other languages

Hello,
What does the "NoIn" as in ExpressionNoIn mean.  I found it here;
http://tomcopeland.blogs.com/EcmaScript.html

Used in the Javascript For loop, opt is a subscript, which does not show it here;
for ( ExpressionNoIn opt ; Expression opt ; Expression opt )

As shown here; http://es5.github.io/  and in the ECMAScript 5.1

I know how the for loop work, no need to explain that   :)
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chima
Asked:
chima
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3 Solutions
 
Dave BaldwinFixer of ProblemsCommented:
It is not something that you can use in your code.  It has something to do with how the language code is designed.  The reference page is here: http://www.ecma-international.org/ecma-262/5.1/  I found some comments where others are as confused about it as you are but nothing about what it actually means.

Here's a page on BNF: http://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form
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Robert SchuttSoftware EngineerCommented:
I know how the for loop work
But do you also know this variant:
var a = ['a', 'b', 'c']; 
for (var i in a)
{
    alert(i + ': ' + a[i]);
}

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On the link Dave posted, it is explained in 12.6.4 and ExpressionNoIn is defined at the very bottom of A.3

So I would say it is used to define that you cannot use the 'in' variant of an expression combined with the 'normal' variant of the for statement (using semicolons). Something quite logical probably but which they had to define in the language with a very elaborate rewrite doubling up a lot of the existing definitions...
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Dave BaldwinFixer of ProblemsCommented:
I had never seen that before.  Tried it, it worked.
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chimaAuthor Commented:
robert_schutt, I understand the code sample you posted, but I can't understand the
http://www.ecma-international.org/ecma-262/5.1/

And about section A.3 - Annex A (informative) Grammar Summary it is just more of the same :)

May I ask you a question about this section?
See >>> below, more than one.
12.6.4 The for-in Statement

The production IterationStatement : for ( LeftHandSideExpression in Expression ) Statement is evaluated as follows:

Let exprRef be the result of evaluating the Expression.
>>> The expession mentioned here is the "far right Expression" in the code/statement...right?

Let experValue be GetValue(exprRef).

If experValue is null or undefined, return (normal, empty, empty).
>>> Why are there three; "normal, empty, empt"  Isn't there only "null and undefined"?

Let obj be ToObject(experValue).

Let V = empty.
>>> What is "V" here? (this is the most important question).

Thank you, your code sample help;
My understanding is that you could not do this;
var a = ['a', 'b', 'c'];
for ( (var in a) ; Expression opt ; Expression opt )

Which does not make sense relative to the other expressions in the For loop.
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Robert SchuttSoftware EngineerCommented:
It's a fairly high level technical document. As Dave already remarked, more people are confused by it. To understand what it all means you have to start at page 1.

I usually get by with a working knowledge of these type of constructs, For me it's more important to know what the for statement does in general than to be able to explain the ECMA definition word for word, which I can't but will try to answer your questions at the end of this comment.

So for me it's most important to understand that with the 'var a' that I posted earlier this code:
for (var i in a)

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is generally equivalent to:
for (var i = 0; i < a.length; i++)

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Now, there are undoubtedly exceptions, that is why that formal definition is so complicated. In the context of that document it is not enough to say they are more or less the same, it has to be defined when they are the same or not. But for me it's often enough to know this is how it works and in this case specifically to know that ExpressionNoIn does indeed mean that it is a syntax error to use something like:
for (var i in a; i < a.length; i++)

Open in new window

but actually your common sense should already tell you that it would not be clear what this would do.

To get back on track, I think the answers to your questions are:
1) yes, in my example: a
2) not sure, describes an exceptional situation
3) Value assigned to be processed in the loop maybe, in my example each value assigned to i??
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tliottaCommented:
One variant can use the "in" operator, and another variant must have no "in" operator. The first is expressed as simply {Expression}. The second is {ExpressionNoIn}. (I.e., {an Expression with no "in"}.)

That's really all it's trying to say.

Tom
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chimaAuthor Commented:
robert_schutt I greatly appreciate your comments and I do get your point.
Allow me to ask tliotta for one code example, if he wishes.

tliotta I think I know what you are saying, would think that the initial statement of;
for ( ExpressionNoIn opt ; Expression opt ; Expression opt )

would be (based on your comment; "The second is {ExpressionNoIn}. (I.e., {an Expression with no "in"}", would be shown as follows:
for ( ExpressionNoIn opt ; ExpressionNoIn opt ; ExpressionNoIn opt )

So, one code example would be greatly appreciated.
Thanks
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tliottaCommented:
...would be shown as follows:
for ( ExpressionNoIn opt ; ExpressionNoIn opt ; ExpressionNoIn opt )


Well, except that isn't strictly correct. The second and third expressions don't have the same requirements as the first. The "in" operator isn't as much of a potential problem in those positions. The ExpressionNoIn expression is only meaningful in the first position.

Tom
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tliottaCommented:
I have to add a disclaimer that my comments are not "authoritative". They're purely from general experience and personal interpretation in case it helps spark useful insights. If I make sense and it helps, it might be coincidence.

Tom
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chimaAuthor Commented:
I apologize for the delay on issuing the points.  Thank you all.
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