How to simplify JavaScript/ECMAScript array litera

问题:

I currently implementing a JavaScript/ECMAScript 5.1 parser with JavaCC and have problems with the ArrayLiteral production.

ArrayLiteral :
    [ Elision_opt ]
    [ ElementList ]
    [ ElementList , Elision_opt ]

ElementList :
    Elision_opt AssignmentExpression
    ElementList , Elision_opt AssignmentExpression

Elision :
    ,
    Elision ,

I have three questions, I'll ask them one by one.

---

I have tried to simplify/to rewrite the ArrayLiteral production depicted above and finally arrived to the following production (pseudo-grammar):

ArrayLiteral:
    "[" ("," | AssignmentExpression ",") * AssignmentExpression ? "]"

My first question: is this rewriting correct?

Two other quetsions:

• LOOKAHEADs for the JavaScript/ECMAScript array literal production
• How to implement a negative LOOKAHEAD check for a token in JavaCC?

回答1:

Yes, that correctly captures the grammar presented.

However, a better rewrite would be:

"[" AssignmentExpression ? ( "," AssignmentExpression ? ) * "]"

because the rewriting in the OP is not LL(1) -- you cannot distinguish the possibilities without reading the entire AssignmentExpression -- whereas with this one you can figure out which alternative to use simply by looking at the first token.