问这个递归的JavaScript函数是如何工作的？

EN

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} val
 * @return {TreeNode}
 */
var searchBST = function(root, val) {
    if(root === null){
        return null;
    }
    else if(root.val === val){
        return root;
    }
    else if(val < root.val){
        return searchBST(root.left, val);
    }
    return searchBST(root.right, val);
};