failed zoom and drag

Here lies the secret to the failure.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

</style>
<svg width="800" height="500"></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 
var 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)
    }; 
});

var rect = svg.append("g")
    .attr("class", "rect")
    .append("rect")
    .attr("width", width)
    .attr("height", height)
    .style("fill", "black")
    .style("pointer-events", "all")
    ;
    
//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", "yellow"); 
        
        
        
//create drag handler with d3.drag()
//only interested in "drag" event listener, not "start" or "end"        
var drag_handler = d3.drag()
    .on("drag", drag_drag);

function drag_drag(d) {
console.log("drag triggered")
  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);    
 
     
//create zoom handler 
var zoom_handler = d3.zoom()
    .on("zoom", zoom_actions);

//specify what to do when zoom event listener is triggered 
function zoom_actions(){
  circles.attr("transform", d3.event.transform);
}

//add zoom behaviour to the rectangle 
//same as rect.call(zoom_handler); 
zoom_handler(rect);

</script>

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