function updateArrayItem(observer, idx, value) {
    observer.set(idx, value);
    this[idx] = value;
}
function updateArrayItem(observer, idx, value) {
    observer.set(idx, value);
    this[idx] = value;
}
function updateArrayItem ( observer , idx , value ) { observer . set ( idx , value ) ; this [ idx ] = value ; }
Update array item value

@param {Observer} observer the observer
@param {number} idx the index to update
@param {*} value the value to set
Update array item value