test

Array.prototype.fillCopy = function(fillValue, start = 0, end = this.length) {
    start = start >=0 ? start : this.length + start;
    end = end >=0 ? end : this.length + end;
    while (start < end) {
        this[start] = fillValue;
        start++
    }
    return this
};
let arr = [1,2,3,4,5,6]
console.log(arr.fillCopy(8, 1))