In Javascript: why does var a, b = 3 return undefined while b = 3 does not?

Question

In the MDN the comma operator is described:

The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand.

But why does

var a, b = 3

return undefined, while the expression

b = 3

will return 3, doesn't it?

Answer 1

This:

var a, b = 3;

is a VariableStatement. VariableStatement evaluation in "normal completion" to empty:

1. Let next be the result of evaluating VariableDeclarationList.
2. ReturnIfAbrupt(next).
3. Return NormalCompletion(empty).

This:

b = 3;

is an ExpressionStatement. ExpressionStatement evaluates to the result of the evaluating expression:

1. Let exprRef be the result of evaluating Expression.
2. Return ? GetValue(exprRef).