Why can click not see update

I am building a d3js based javascript frontpage. Whilst restructuring some code from functional to OOP the click method stopped being able to find the update method. I get Uncaught ReferenceError: this.update is not defined. I think its something to do with scope but im quite new to js so its hard for em to figure it out, any ideas?

   class UpdateTree{

       update(source){}

       enter_new_nodes_at_the_parents_previous_position(node, source){
           var nodeEnter = node.enter().append("g")
             .attr("class", "node")
             .attr("transform", function (d) { return "translate(" + source.y0 + "," + source.x0 + ")"; })
             .on("click", this.click);
      }

       click(d) {

      if (d.children) {
          d._children = d.children;
          d.children = null;
      } else {
          d.children = d._children;
          d._children = null;
      }
      this.update(d);
    }
}

Answer

Reading more into this problem myself I have found that whilst javascript can be both functional and object orientated, d3js is (says this on their wiki) based around functional programming, therefore it seems to be a mistake to try an make it OOP.