erik.js

function longestConsecutive(nums) {
        let longestConsecutiveLength = 0,
            longestKey;         // FOR TESTING: This stores the key of a number in the longest sequence, used to build the list for display

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

        /* FOR TESTING: To build the list of consecutive numbers to display */
        const consecutiveList = [longestKey];
        function buildConsecutiveList(key, direction) {
            const neighbor = getNeighbor(key, direction);
            if (neighbor !== false) {
                consecutiveList.push(neighbor);
                buildConsecutiveList(neighbor, direction);
            }
        }
        buildConsecutiveList(longestKey, NEIGHBOR_ABOVE);
        buildConsecutiveList(longestKey, NEIGHBOR_BELOW);

        return longestConsecutiveLength;
    }