JSFiddle - React, Tailwind, and code Playground

JavaScript

var items = {
  "5": ["name", "link2"],
  "8": ["name 2", "link 2"],
  "11": ["name 3", "link 3"]
};

var getItem = function(key, i) {
  var keys = Object.keys(items).sort(function(a,b){return a-b;});
  var index = keys.indexOf(key);
  if ((i==-1 && index>0) || (i==1 && index<keys.length-1)) {return items[keys[index+i]];}
  else {return (i==1?"last":"first")+" item";}
}

console.log(getItem("5",-1));
console.log(getItem("11",+1));