Are "for...in" loops in Javascript always bad?

Question

I have a situation that I'm trying to make work with an array, but I just don't think it will. I want to have a list of objects, each with a unique ID, and I want to be able to easily reference a particular object without having to loop through the array searching for that ID.

If I use an object I can easily use the unique ID as the key and the object as the value. However, if I use an object instead of an array I'll have to use a for...in loop, and I know there are issue surrounding that if someone using my code has extended Object.prototype.

So here's my question:

Is it really that common that people extend Object.prototype that I should be weary of using it? Are there other reasons why I wouldn't want to use a for...in loop?

On the other hand, is the performance hit of looping through an array looking for a unique ID so minimal that I shouldn't worry about?

(For the record, the array will probably have fairly few elements in it, but I'll be accessing it very frequently. Also, the code I'm writing will be a jQuery plugin, so I have no control over what other bad code people are combining it with.)

UPDATE:

Based on advice from @nnnnnn, I set up a jsperf test and here are the results: http://jsperf.com/accessing-object-properties-vs-iterating-over-arrays

Basically, though the object way is slightly faster, the difference is negligible. Still, using for...in with hasOwnProperty seems cleaner.

Answer 1

Even if someone does extend the prototype, you can use hasOwnProperty property to exclude these members. This is exactly what Sugar.js recommends:

var s = 'cat', key;
for(key in s) {
  if(s.hasOwnProperty(key)) {
    console.log(key);
  }
}

Now according to Sugar, for..in is the accepted loop for object literals, but you can also use an iterator like $.each for jQuery or _.each for Underscore.js.