我可以减少这个 Javascript 代码吗？

Question

Can I reduce

function n()
  {
  var a; 
  if(a = document.getElementById('fb0')) 
    {
    a.onclick = i0;
    document.getElementById('fb1').onclick = i1;
    }
  }

to

function n()
  {
  if(document.getElementById('fb0').onclick = i0) 
    {
    document.getElementById('fb1').onclick = i1;
    }
  }

I don't have a debugger right now. I know that document.getElementById('fb0') returns a value because the first snippet works fine. But does it need the assignment to be evaluated in the if statement?

Answer 1

Not really; if getElementById('fb0') doesn't return anything your page will get an error, and in the first case it wouldn't.

Answer 2

To check if "document.getElementById('fb0')" returns an element or null, the second version don't do it and an error will be throw if there is no element with id "fb0". The second version is ok if you don't remove the "fb0" element from the DOM at some point.

Answer 3

No, you can't.

document.getElementById('fb0') , as the function name already says, returns the html element with has the id equal to fb0 . After that you are accessing the attribute onclick . But it the get fails it will break the script.

On the first scenario you test if the assignment works, if does it means the element exists and will only execute if it exists.

Those are different behaviors.

Answer 4

That would fail if document.getElementById('fb0') were not to exist. document.getElementById('fb0').onclick wouldn't mean much in that case.

Answer 5

If you do a lot of DOM selection by ID, make a shortened version of that method:

function byId( id ) { return document.getElementById(id); };

function n() {
  var a; 
  if(a = byId('fb0')) {
    a.onclick = i0;
    byId('fb1').onclick = i1;
  }
}

In this case, doing the assignment inside the condition doesn't save you any characters. It's the same length to do it with the declaration.

function byId( id ) { return document.getElementById(id); };

function n() {
  var a = byId('fb0'); 
  if(a) {
    a.onclick = i0;
    byId('fb1').onclick = i1;
  }
}