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

I've split the code into smaller parts with inline comments for better understanding.

var word = "AAAABBBBCCBB";
var uniqueElements = function(word){
    // this is the filter function
    // this function will get each element of the string with its index
    var filterFunction = function(elem, index) {
        // the elements who pass the following condition are kept
        // the condition - 
        // if the character at the previous index (index-1) is not equal to the 
        // current element, then keep the element
        return word[index-1] !== elem;
    }

    // the below is equivalent to Array.prototype.filter.call(context, filterFunction)
    return [].filter.call(word, filterFunction);
}
console.log(uniqueElements(word));