Question

What are the differences of that iterations:

var recordId;
for(recordId in deleteIds){
  ...
}

and

for(var recordId in deleteIds){
  ...
}

It says implicit definition(what is it), is there a performance difference between them?

Answer 1

the two samples are equivalent, however the first may come from following a recommended pattern in JavaScript which is declaring all variables at the top of every function.

Sample:

var recordId,
    i = 0;

for(recordId in deleteIds){
  ...
  i++;
}

More explanation on this can be found here JSLint error: Move all 'var' declarations to the top of the function