Dragging Circles

Example of using version 4 of d3.js to drag circles around the screen. Want to learn this? Read this.

<!DOCTYPE html>
<meta charset="utf-8">
<svg width="960" height="600"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>
var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height");
    
    
//create some circles at random points on the screen

//create 50 circles of radius 20
//specify centre points randomly through the map function 
radius = 20;
var circle_data = d3.range(50).map(function() {
    return{
        x : Math.round(Math.random() * (width - radius*2 ) + radius),
        y : Math.round(Math.random() * (height - radius*2 ) + radius)
    }; 
});

//add svg circles 
var circles = d3.select("svg")
	.append("g")
	.attr("class", "circles")
	.selectAll("circle")
        .data(circle_data)
        .enter()
        .append("circle")
        .attr("cx", function(d) {return(d.x)})
        .attr("cy", function(d) {return(d.y)})
        .attr("r", radius)
        .attr("fill", "green"); 
        
        
        
//create drag handler with d3.drag()
//only interested in "drag" event listener, not "start" or "end"        
var drag_handler = d3.drag()
    .on("drag", function(d) {
          d3.select(this)
            .attr("cx", d.x = d3.event.x  )
            .attr("cy", d.y = d3.event.y  );
            }); 
      
      
//apply the drag_handler to our circles 
drag_handler(circles);

</script>

https://d3js.org/d3.v4.min.js