[рони]

Evelate,

function clearArrayFromUndefined(arr){
                for (var j = 0; j < arr.length; ) {
                    var n = arr[j];
                    if (n === undefined) {
                        for (var k = j; k < arr.length-1; k++) {
                            arr[k] = arr[k+1];
                        }
                        arr.length--;
                    }
                    else j++
                }
    return arr;
}


var array = [];

array[0] = 0;
array[1] = 1;

array[3] = 3;

array[5] = 5;
array[8] = 8;
array[15] = 15;
array[20] = 20;
//array[50] = 50;

console.log(array);
clearArrayFromUndefined(array);
console.log(array);