binary-search-tree-check(js) using In-Order Traversal method

according to article in http://www.geeksforgeeks.org/a-program-to-check-if-a-binary-tree-is-bst-or-not/

the current binary search tree check using the METHOD 1 (Simple but Wrong). if tree is like:
```javascript
var bstfalse = {
  value: 3,
  left: {
    value: 2,
    right: {
      value: 10
    }
  },
  right: {
    value: 5
  }
};
```
The current result is true,which should be false;

so I rewrite the code regarding the article and using METHOD 4(Using In-Order Traversal)

```javascript
module.exports = function(bst) {
  if (!bst || bst.value === null) {
    return false;
  }

  var prev;

  return (function isBst(root) {
    if ( !! root) {
      if (!isBst(root.left)) {
        return false;
      }

      if (prev && prev.value >= root.value) {
        return false;
      }

      prev = root;

      return isBst(root.right);
    }

    return true;
  })(bst);
};

```