void（0）返回`undefined`，但允许属性访问。为什么？

Question

So void returns undefined after executing the expression passed to it. undefined throws exceptions when you try to access its properties. So why is that void(0).prop returns undefined instead of crashing?

alert("void(0) => " + void(0)); // undefined

// How is it that this doesn't throw an exception?
alert("void(0).someprop => " + void(0).someprop); // undefined

// Exception, can't access property of undefined.
alert("undefined.someprop => " + undefined.someprop); // crash

http://jsfiddle.net/bFhLS/

Answer 1

The void operator doesn't use parenthesis itself. So, the statement is probably being parsed as:

void( (0).someprop )

And accessing someprop from the Number . Rather than as:

(void (0)).someprop

As you were probably expecting, which does throw an error.

Answer 2

void is an operator , it is NOT a function.

void(0) is equivalent to "void 0" .

So void(0).someprop is equivalent to void 0..someprop .

To prove,

void(undefined).someprop 

throws an error, since it will be evaluated as ( someprop doesn't exist in undefined for sure)

void undefined.someprop