CODEWITHSUNDEEP
javascriptecmascript-6context-free-grammarcompiler-construction
Max Koretskyi
Max Koretskyi

Reputation: 105497

How can `AssignmentExpression` be derived from `Statement` in EcmaScript grammar

Suppose I have the following part of JS code:

const v = 3;

As I understand it can be parsed as AssignmentExpression:

AssignmentExpression :
    LeftHandSideExpression = AssignmentExpression

Now I'm wondering how it can be derived from the Statement? One possible route is:

Statement -> ExpressionStatement -> Expression -> AssignmentExpression

But I'm not sure about that. Is it correct?

Here is how I found it:

AssignmentExpression is part Expression:

Expression :
    AssignmentExpression
    Expression, AssignmentExpression

then Expression is part of ExpressionStatement is 

ExpressionStatement:

    [lookahead ∉ { {, function, ..., let [ }] Expression;

and then ExpressionStatement is part of Statement:

Statement :
    ExpressionStatement

Upvotes: 4

Views: 487

Answers (1)

sepp2k
sepp2k

Reputation: 370172

const v = 3; is a declaration, not an assignment. Interestingly declarations aren't Statements, but can appear in StatementLists. So you can't derive const v = 3; from Statement, but you can derive it from StatementList. So if you have a script containing only const v = 3;, the full derivation would be:

            Script
               |
          ScriptBody
               |
         StatementList
               |
       StatementListItem
               |
          Declaration
               |
      LexicalDeclaration
     /         |        \
LetOrConst BindingList  ';'
    |           |
 'const'   LexicalBinding
             /          \
     BindingIdentifier  Initializer
            |                |
         Identifier    AssignmentExpression
            |                  |
       IdentifierName         ...
            |                  |
           'v'                '5'

An actual assignment would be just v = 5;. That would go through ExpressionStatement and the derivation from Statement would look like this:

                     Statement
                         |
               ExpressionStatement
                     /        \
                   Expression ';'
                        |
              AssignmentExpression
             /          |         \
LeftHandSideExpression '=' AssignmentExpression
         |                         |
    NewExpression                ...
         |                         |
   MemberExpression               '5'
         |
   PrimaryExpression              
         |
IdentifierReference
         |
     Identifier
         |
   IdentifierName
         |
        'v'

Upvotes: 6

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