firstRecurringCharacter

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]));