为什么不能使用“ !! document.getElementsByClassName && function（）{return document.getElementsByClassName（obj）}”简化代码

Question

when I want to check whether getElementsByClassName exists for Explorer. I use under code and it's working well.

 function getClass(obj) { if (document.getElementsByClassName) { return document.getElementsByClassName(obj) } }//work well 

for simplify my codes, I change codes and work well also:

 function getClass(obj) { if (document.getElementsByClassName) return document.getElementsByClassName(obj); }//work well 

But when I write this , it gives some error

 function getClass(obj) { !!document.getElementsByClassName && ( return document.getElementsByClassName(obj)); }//work badly 

There is a error within Chrome.Error message is "Uncaught SyntaxError: Unexpected token return". for cure this error,I change my codes ,see under:

 function getClass(obj) { !!document.getElementsByClassName && function() { return document.getElementsByClassName(obj) }; } 

BUT,when I use getClass() ,there console show "undefined".

for description above,

1. I don't know why "&&" used so hard?
2. I want to know is there has some simplify code?

Answer 1

The problem with this one:

function getClass(obj) {
    !!document.getElementsByClassName && ( return document.getElementsByClassName(obj) );
}//work badly

You can't use return as part of a logic expression.

The problem with this one:

function getClass(obj) {
    !!document.getElementsByClassName && function() {
        return document.getElementsByClassName(obj)
    };
}

You don't actually return anything, you just create an anonymous function that is not assigned to anything, so the default undefined is always returned.

You could do something like this by putting return before the logic conditions.

function getClass(obj) {
    return !!document.getElementsByClassName && document.getElementsByClassName(obj);
}

Honestly though, these short circuit tricks really aren't simpler, and I would recommend just using option 1 or perhaps the ternary operator.

function getClass(obj) {
    return document.getElementsByClassName ?
        document.getElementsByClassName(obj) :
        null;
}