// Replace 1 array element at index with item 

arr.splice(index,1,item);

array.splice(array.indexOf(valueToReplace), 1, newValue);

function solution(inputArray, elemToReplace, substitutionElem) {

    return inputArray.map((item) => {

        if (item === elemToReplace) {

            return substitutionElem;

        }

        return item;

    });

}

function solution(inputArray, elemToReplace, substitutionElem) {

    return inputArray.map((x) => (x === elemToReplace ? substitutionElem : x));

}

String.prototype.replaceArray = function(find, replace) {

  var replaceString = this;

  for (var i = 0; i < find.length; i++) {

    replaceString = replaceString.replace(find[i], replace[i]);

  }

  return replaceString;

};

var array1 = ['S', 'T', 'A', 'C', 'K', 'O', 'V', 'E', 'R', 'F', 'L', 'O', 'W'];

​

array1.fill('X', 3, 10)

​

console.log(array1)