Choropleth Map application graphed in D3.js.
This project is based on FreeCodeCamp Project 4
Source Code

The top few things I learned

1
I need the TopoJSON library to work with geographic data.
Each County and State is a path.

2
How do I access the result of all the Promises in:

			Promise.all(promises).then(main);

			

The solution is:

			function main(data){ 
			    var dataset = data[0];
			    var dataset2 = data[1];
			    ...
			}

			

3
Problem:
The tooltip sometimes did not appear when I moved it to adjacent counties.
This was because on mouseout, the tooltip opacity is set to 0 but tooltip was still there but not visible.
This interfered with displaying the subsequent tooltip.
Solution:
Move tooltip above and away from the cursor pointer.

			//tooltip appears above cursor
			tooltipDiv.html(tooltipData)
			.style("left", d3.event.pageX + "px")
			.style("top", d3.event.pageY - 50 + "px");