ECMAScript AssignmentExpression 生产规则如何产生`foo = 42`？

Question

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?

Answer 1

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.