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

i have task receive unique element in order string parameter. not understand how function uniqueelements returns ['a','b','c','b'].

var word = "aaaabbbbccbb"; var uniqueelements = function(word) {     return [].filter.call(word, (function(elem,index) {         return word[index-1] !== elem     })); }     uniqueelements(word);       //it should return ['a','b','c','b'] 

i checked , read following sources:

mdn mozilla , js blog without success.

it's performing explicit call() on filter method on array object can pass in string. they're using [] instead of array because it's shorter syntax or trying clever when wrote it.

normally, filter() method affect array's own contents. however, call()-ing it, can change context passing in array, or in case, string (which treated array of characters). runs supplied function each element, keeping element if return value of function true.