sum() {

    let summ= 0

    let traverse = (node) => {

        if (node.left) {

         summ =summ + node.left.data;

          traverse(node.left);

        }

​

        if (node.right){

          summ =summ + node.right.data;

          traverse(node.right);}

    }

    traverse(this.root);

    summ = summ+this.root.data

    return summ;

  }

​

//if you find this answer is useful ,

//upvote ⇑⇑ , so can the others benefit also . @mohammad alshraideh ( ͡~ ͜ʖ ͡°)