Im trying to build an AVL/Binary tree, but was not able to find a lot of code examples, only theory. My implementation for some reason has the AVL rotation function losing half the tree when the tree is 1 sided.

Here is my code:

function buildTree(dataSet){

    let root = null

    function rotateRight(node){
        return rotate(node,true)
    }
    function rotateLeft(node){
        return rotate(node,false)
    }
  
    function rotate(node,right){
    
    const  inputNodeSide = right ? "left"  : "right"
    const targetNodeSide = right ? "right" : "left"

    const targetNode      =       node[ inputNodeSide]
    const targetNodeChild = targetNode[targetNodeSide]
    
    targetNode[targetNodeSide] = node
          node[ inputNodeSide] = targetNodeChild
    
    return targetNode // as this is at the top
  }
  
function createNode(data){

    return {
    data,
    left : null, 
    right : null,
    get balance(){
        return workOutHeight(this.left) - 
             workOutHeight(this.right)
    },
    get height(){
        return workOutHeight(this)
    },
  }
} // END createNode

  function workOutHeight(node){
    if(null === node){
        return -1
    }
    return Math.max(workOutHeight(node.left),
                    workOutHeight(node.right))+1
  }
  
  function avl(node){
    const balanced = node.balance
    if(2 === balanced){
      if(0 > node.left.balance){
        node.left = rotateLeft(node.left);
      }
      return rotateRight(node); 
    } else if(-2 === balanced){
      if(0 < node.right.balance){
        node.right = rotateRight(node.right);
      }
      return rotateLeft(node); 
    }
    return node
  }
  
   this.add = function(data, perent){
    perent = perent || root;
    if(null === perent){
        return root = createNode(data);
    } else if(data < perent.data){
      if(null === perent.left){
        return perent.left = createNode(data);
      }
      this.add(data,perent.left)
      avl(perent)
    } else if(data > perent.data){
      if(null === perent.right){
       return perent.right = createNode(data);
      } 
      this.add(data,perent.right)
      avl(perent)
    }
  } // END addData
  
  this.tree = function(){
    return JSON.parse(JSON.stringify(root))
  }
  
  if(Array.isArray(dataSet)){
    dataSet.forEach(val=>this.add(val))
  }

} // END buildTree

console.log(new buildTree([2,6,9,4,7,0]).tree())

Thanks for any help on this.