how to find tree is a valid binary tree code example

Example: check if binary search tree is valid

class BTNode {
  constructor(value) {
    this.value = value;
    this.left = null;
    this.right = null;
  }
}

/**
 *
 * @param {BTNode} tree
 * @returns {Boolean}
 */
const isBinarySearchTree = (tree) => {
  if (tree) {
    if (
      tree.left &&
      (tree.left.value > tree.value || !isBinarySearchTree(tree.left))
    ) {
      return false;
    }
    if (
      tree.right &&
      (tree.right.value <= tree.value || !isBinarySearchTree(tree.right))
    ) {
      return false;
    }
  }
  return true;
};