Question

I have this:

var field = query("form#"+form.id+" input[name='" + error.field + "']");
if(field.length){
  if( field[0].id ) {
    widget = registry.byId( field[0].id );
    if(widget){
    ...
  }
 }

I would have thought I could write:

var field = query("form#"+form.id+" input[name='" + error.field + "']");
if(field.length && field[0].id && widget = registry.byId( field[0].id) ){
...
}

But if I use the second, “shortened” form, I get a Javascript error.

Sorry, it would be messy to give a JSFiddle. The problem happens when query() returns 0 values — field[0].id && widget = registry.byId( field[0].id) were still interpreted.

I thought that I was playing it safe as the second field[0].id would only happen if field.length was > 0 and then widget = registry.byId( field[0].id) would only happen if field[0].id is true…

What am I missing?

Merc.

Answer 1

You may be getting Uncaught ReferenceError: Invalid left-hand side in assignment error which is because you are trying to evaluate registry.byId() method on RIGHT and assigning it into result on LEFT but as you’ve not restricted your left-hand assignment using brackets, the entire field.length && field[0].id && widget is considered which raised an error.

You can suppress the error by wrapping your last condition in brackets.
if(field.length && field[0].id && (widget = registry.byId( field[0].id)) ){