Path Sum JS DFS

var pathSum = function(root, targetSum) {
    let sumCount = new Map([[0, 1]]), ans = 0;

    const dfs = (root, prevSum) => {
        if (root === null) return;
        const sum = prevSum + root.val;

        ans += sumCount.get(sum - targetSum) || 0;
        sumCount.set(sum, (sumCount.get(sum) || 0) + 1);

        dfs(root.left, sum);
        dfs(root.right, sum);
        sumCount.set(sum, sumCount.get(sum) - 1);
    };

    dfs(root, 0);
    return ans;
};