CODEWITHSUNDEEP
javascriptecmascript-5specificationsassignment-operator
szcsl
szcsl

Reputation: 399

How can ECMAScript AssignmentExpression production rule produce `foo = 42`?

As the spec states (yes, 5.1, that's intentional):

AssignmentExpression :

    ConditionalExpression
    LeftHandSideExpression = AssignmentExpression
    LeftHandSideExpression AssignmentOperator AssignmentExpression

Based on this definition, I don't understand how a simple foo = 42 assignment is valid.

My understanding is, that foo = should always be followed by either a ConditionalExpression or essentially another assignment, but in that case, how will the production terminate if I don't use a ConditionalExpression? All further AssignmentExpression rules require either a ConditionalExpression or another AssignmentExpression.

What am I missing here?

Upvotes: 4

Views: 86

Answers (1)

Bergi
Bergi

Reputation: 664599

The expression after the = operator must always be an AssignmentExpression. This AssignmentExpression will often consist not of another assignment, but a plain ConditionalExpression, which in turn will consist of a simpler expression that might contain an operator or not, all the way down through the operator precedence hierarchy to the Literal that your number 123 forms.

Upvotes: 1

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