Range Sum of BST.js

// Runtime: 248 ms (Top 56.51%) | Memory: 95.8 MB (Top 75.37%)
var rangeSumBST = function(root, low, high) {
    let sum = 0;
    const summer = (root) => {
        if(!root) {
            return;
        }

        sum = root.val >= low && root.val <= high ? root.val + sum : sum;

        summer(root.left)
        summer(root.right)
    }

    summer(root);

    return sum;
};