d3 v4: merge enter and update selections to remove duplicate code

Question

I understand that merge can be used to combine enter and update selections in d3 v4, as in the simple example here: https://bl.ocks.org/mbostock/3808218.

I have a scatter plot in which multiple variables are displayed on a shared x-axis, for different groups selected by a dropdown box. When a new group is selected, the overall set of datapoints is updated, with points for each variable added like this:

.each(function(d, i) {
        var min = d3.min(d.values, function(d) { return d.value; } );
        var max = d3.max(d.values, function(d) { return d.value; } );

    // Join new data with old elements
        var points = d3.select(this).selectAll("circle")
          .data(d.values, function(d) { return (d.Plot); } );

    // Add new elements
        points.enter().append("circle")
          .attr("cy", y(d.key))
          .attr("r", 10)
          .style("opacity", 0.5)
          .style("fill", function(d) { return elevColor(d.Elevation); })
          .merge(points) //(?)
          .transition()
          .attr("cx", function(d) { return x((d.value-min)/(max-min)); });

    // Remove old elements not present in new data
        points.exit().remove();

This whole piece of code is largely duplicated for the overall enter selection and again in the overall update selection (as opposed to the individual variables), which seems less than ideal. How would merge be used to to remove this duplicated code?

The full example is here: http://plnkr.co/edit/VE0CtevC3XSCpeLtJmxq?p=preview

Answer 1

I'm the author of the solution for your past question, which you linked in this one. I provided that solution in a comment, not as a proper answer, because I was in a hurry and I wrote a lazy solution, full of duplication — as you say here. As I commented in the same question, the solution for reducing the duplication is using merge.

Right now, in your code, there is duplication regarding the setup of the "update" and "enter" selections:

var update = g.selectAll(".datapoints")
    .data(filtered[0].values);

var enter = update.enter().append("g")
    .attr("class", "datapoints");

update.each(function(d, i){
    //code here
});

enter.each(function(d, i){
    //same code here
});

To avoid the duplication, we merge the selections. This is how you can do it:

var enter = update.enter().append("g")
    .attr("class", "datapoints")
    .merge(update)
    .each(function(d, i) {
        //etc...

Here is the updated Plunker: http://plnkr.co/edit/MADPLmfiqpLSj9aGK8SC?p=preview