ecmascript规范的语法令人困惑

Question

I am confusing on the following specification rules. Why is an "AdditiveExpression" should consist of another "AdditiveExpression" and "MultiplicativeExpression"?

As the following rules say, we should evaluate the "AdditiveExpression" on the left side and the "MultiplicativeExpression" on the right side first, but what should the rule do if the right side of the operand is not the evaluating of "MultiplicativeExpression" but a "AdditiveExpression"?

Answer 1

The syntax says the right operand of + must be a MultiplicativeExpression. If it isn't, you get a syntax error and nothing is evaluated.

For example, the expression 1 + 2 + 3 is parsed like this:

• 1 is a NumericLiteral , which is a Literal , which is a PrimaryExpression .

• A PrimaryExpression is a MemberExpression , which is a NewExpression , which is a LeftHandSideExpression , which is an UpdateExpression , which is a UnaryExpression , which is an ExponentiationExpression , which is a MultiplicativeExpression .

• A MultiplicativeExpression is an AdditiveExpression .

• + is ... + .

• 2 is a NumericLiteral , which is (by the chain listed above) a MultiplicativeExpression .

• At this point the rule AdditiveExpression : AdditiveExpression + MultiplicativeExpression (quoted in your question) matches: 1 + 2 is parsed as an AdditiveExpression .

• We have another + , which is just + .

• 3 is a NumericLiteral , which is (by the chain listed above) a MultiplicativeExpression .

• Now AdditiveExpression : AdditiveExpression + MultiplicativeExpression applies again (with 1 + 2 being the AdditiveExpression and 3 being the MultiplicativeExpression ).

• The whole thing is now an AdditiveExpression .

Answer 2

Why does the "AdditiveExpression" should consist of another "AdditiveExpression" and "MultiplicativeExpression"?

So that you can nest chain additive expression indefinitely: 1 + 1 + 1 + 1 + …

what should the rule do if the right side of the operand is not the evaluating of "MultiplicativeExpression" but a "AdditiveExpression"?

That just doesn't happen. The expression 1 + 1 + 1 does parse as (1 + 1) + 1 (where the right side is a MultiplicativeExpression), not as 1 + (1 + 1) .

The single number can be parsed as a "MultiplicativeExpression"?

Yes. The single number on the right side is a multiplicative expression, the single number on the lift side is an addititive expression :

AdditiveExpression[Yield, Await]: MultiplicativeExpression[?Yield, ?Await]

that is also a multiplicative expression , which is an ExponentiationExpression , which is an UnaryExpression , which is a UpdateExpression which is a LeftHandSideExpression , which is a NewExpression , which is a MemberExpression , which is a PrimaryExpression , which is a Literal , which consists of NumericLiteral lexem.

This kind of nesting is how operator precedence is determined in the JS grammar.