erik.js

const tracker = {};
const NEIGHBOR_ABOVE = "above";
const NEIGHBOR_BELOW = "below";

function updateConnected(current, value, direction) {
    const neighbor = getNeighbor(current, direction);
    if (neighbor !== false) {
        tracker[neighbor] = value;
        updateConnected(neighbor, value, direction);
    }
}
function getNeighbor(current, direction) {
    if (direction === NEIGHBOR_ABOVE) {
        neighbor = current + 1;
    } else if (direction === NEIGHBOR_BELOW) {
        neighbor = current - 1;
    } else {
        console.error("Unexpected direction");
    }

    return (tracker[neighbor] !== undefined ? neighbor : false);
}

function longestConsecutive(nums) {
    let longestConsecutiveLength = 0;

    for (let i = 0; i < nums.length; i++) {
        let currentValue = nums[i],
            neighborAbove = getNeighbor(currentValue, NEIGHBOR_ABOVE),
            neighborBelow = getNeighbor(currentValue, NEIGHBOR_BELOW),
            aboveConsecutiveLength = (tracker[neighborAbove] !== undefined ? tracker[neighborAbove] : 0),
            belowConsecutiveLength = (tracker[neighborBelow] !== undefined ? tracker[neighborBelow] : 0),
            newConsecutiveLength = aboveConsecutiveLength + 1 + belowConsecutiveLength;

        tracker[currentValue] = newConsecutiveLength;
        if (aboveConsecutiveLength > 0) {
            updateConnected(currentValue, newConsecutiveLength, NEIGHBOR_ABOVE);
        }
        if (belowConsecutiveLength > 0) {
            updateConnected(currentValue, newConsecutiveLength, NEIGHBOR_BELOW);
        }
        if (newConsecutiveLength > longestConsecutiveLength) {
            longestConsecutiveLength = newConsecutiveLength;
            longestKey = currentValue;
        }
    }

    return longestConsecutiveLength;
}