Who can help to explain this JavaScript algorithm [].filter.call()

Question

I have task to receive unique element in order from string as parameter. I do not understand how this function uniqueElements returns [\'A\',\'B\',\'C\',\'B\']

Answer 1

If you don't understand what a code do, you can log actions ! IE

var word = "AAAABBBBCCBB";
function uniqueElements(word){
    return [].filter.call(word,(
        function(elem,index) { 
        console.log("Index : "+index+" | Elem : " +elem+" | CompareTo "+word[index-1]);
            return word[index-1] !== elem;
        }
    ));
}
uniqueElements(word); 

You will get the following input :

Index : 0 | Elem : A | CompareTo undefined
Index : 1 | Elem : A | CompareTo A
Index : 2 | Elem : A | CompareTo A
Index : 3 | Elem : A | CompareTo A
Index : 4 | Elem : B | CompareTo A
Index : 5 | Elem : B | CompareTo B
Index : 6 | Elem : B | CompareTo B
Index : 7 | Elem : B | CompareTo B
Index : 8 | Elem : C | CompareTo B
Index : 9 | Elem : C | CompareTo C
Index : 10 | Elem : B | CompareTo C
Index : 11 | Elem : B | CompareTo B

Using this, you can check that every element who aren't equal than the one before are send to your array.

Few answers/comments already pointed out how [].filter.call() work.

If you want to get a String instead of an array, simply add .join("") IE

var word = "AAAABBBBCCBB";
function uniqueElements(word){
    return [].filter.call(word,(
        function(elem,index) { 
        console.log("Index : "+index+" | Elem : " +elem+" | CompareTo "+word[index-1]);
            return word[index-1] !== elem;
        }
    )).join("");
}
uniqueElements(word);