Question

I am trying to boost my Javascript understanding, so I’ve been looking through the Zepto library. I came across this line:

uniq = function(array){
    return array.filter(function(item, idx){
        return array.indexOf(item) == idx
    })
}

What is the purpose of this function? From what I can tell, it is creating a new, unique array of elements, right? But isn’t it essentially just cloning the array? If so, wouldn’t array.slice() be faster?

Finally, wouldn’t it increase performance to change array.indexOf(item) to array.indexOf(item,idx)? Or better yet, just return true? When does array.indexOf(item)==idx not equal true? Is this to prevent duplicate items? But when would that ever actually happen?

Answer 1

it is creating a new, unique array of elements, right?

It just filter your array elements to return unique elements.

demo

But isn't it essentially just cloning the array?

No as I explain above.

If so, wouldn't array.slice() be faster?

Slice doesn’t remove duplicates.

Finally, wouldn't it increase performance to change array.indexOf(item) to array.indexOf(item,idx)? Or better yet, just return true?

If you only return true you won’t identify if the element is duplicated or not.

demo

When does array.indexOf(item)==idx not equal true?

Example:
I have the following array:

['10', '20', '30', '20', '10']

Iterations:

• 1: array.IndexOf(10) == 0 ? // yes, so return true
• 2: array.IndexOf(20) == 1 ? // yes, so return true
• 3: array.IndexOf(30) == 2 ? // yes, so return true
• 4: array.IndexOf(20) == 3 ? // no because array.indexOf(20) is 1 , so return false
• 5: array.IndexOf(10) == 4 ? // no because array.indexOf(10) is 2 , so return false

So, when the element has already been found it gets false because the indexes are not the same.