Merge Sorted Arrays

// Given an array, return the first recurring number
function firstRecurringNumber(arr) {
    if (!arr || arr.length === 0) {
        return undefined;
    }
    
    const numbers = new Map();
    for (const item of arr) {
        if (!numbers.get(item)) {
            numbers.set(item, 1);
        } else {
            return item;
            // numbers.set(item, numbers.get(item) + 1);
        }
    }
    return undefined;
}

console.log(firstRecurringNumber([2,5,1,2,3,5,1,2,4]));
console.log(firstRecurringNumber([2,1,1,2,3,5,1,2,4]));
console.log(firstRecurringNumber([2,3,4,5]));
console.log(firstRecurringNumber([]));