その数に1を加えます(リンクリストで包まれます).

Input1:  [7, 8, 9, 9]
Output1: [7, 9, 0, 0]

//function converting nums in linkedList
function getLinkedList ( ...arr ) {
    let nodesArray = [];
    for( var index=0; index<arr.length; index++ ) {
        nodesArray.push({
            digit: arr[index],
            next:null
        })
    }
    for( var index = 0; index < nodesArray.length-1; index++ ) {
        nodesArray[index].next = nodesArray[index+1];
    }
    if(nodesArray.length){
        return nodesArray[0];
    }
}
//prints the linkedList's digits in an Array.
function printList(list) {
    let arr = [], node = list;
    while(node != null) {
        arr.push(node.digit);
        node = node.next;
    }
    console.log(arr);
}
let list = getLinkedList(7, 8, 9, 9);

printList(list); // [7, 8, 9, 9]

let num =1;//number to be added

//function which will add one to the number
function addOne(_list) {
    //to achieve backtracking calling function before implementation till end
    if(_list.next != undefined){
        addOne(_list.next);
    }
    _list.digit += num;
    //borrow will be in case of sum 10 or greater than ten
    if(_list.digit>10){
        _list.digit = 10-_list.digit;
        num = 1;
    } else if(_list.digit=== 10){
        _list.digit = 0;
        num = 1;
    } else {////borrow will be zero when sum is less than 10
        num =0;
    }
    //to handle all nine case, 
    //example in case of 9999, answer will be 10000
    if(list === _list && num!= 0 ){
        //creating new node for the last borrow (1), in case of all nine
        list = {
            digit : num,
            next: list
        }
    }
}
addOne(list);
printList(list); // [7, 9, 0, 0]
//in case of[9, 9, 9, 9], the output is [1,0,0,0,0]