Check if a BST is valid

Validate a BST

function isBST(root) {
    return validate(root, Number.MIN_SAFE_INTEGER, Number.MAX_SAFE_INTEGER);
}

function validate(node, min, max) {
    if (node === null) {
        return true;
    }

    if (node && node.value < min) {
        return false;
    }

    if (node && node.value > max) {
        return false;
    }

    return validate(node.left, min, node.value) 
        && validate(node.right, node.value, max);
}

function Node(value, leftNode, rightNode) {
    this.value = value;
    this.left = leftNode;
    this.right = rightNode;
}

var n1 = new Node(5, null, null),
    n2 = new Node(1, null, n1),
    subtree = new Node(2, n2, null),
    invalidTree = new Node(10, subtree, null),
    ln = new Node(1, null, null),
    rn = new Node(3, null, null),
    validateTree = new Node(2, ln, rn);

console.log('is a BST: ', isBST(invalidTree), ', this is not a BST');

console.log('is a BST: ', isBST(validateTree), ', this is a BST');