function firstRecurringCharacter(arr) { let recurringIndex = null, recurringValue = null; const obj = {}, len = arr.length; for(let i=0;i < len;i++) { if(obj[arr[i]] === undefined) { obj[arr[i]] = i; }else { if(recurringIndex === null || obj[arr[i]] < recurringIndex) { recurringValue = arr[i]; recurringIndex = obj[arr[i]]; } } } return recurringValue; } console.log(firstRecurringCharacter([2,5,5,2,3,4,5,6,7]));