Rectangles Update Function

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>

<body>
  <svg width="960" height="500"></svg>
  <script>
    var t = d3.transition()
        .duration(500)
    
    function render(data){
    var rects = d3.select("svg")
    						.selectAll("rect")
                .data(data)
                
    rects
    				    .exit()
    						.style("fill","red")  
  //    					.transition().duration(1000)
    						.remove()
    rects
                .enter()
                .append("rect")
                   .attr("x",function(d){return d;})
                   .attr("y", 100)
                   .attr("width", 70)
								   .attr("height",300)
    							 .style("fill","green")
    							 .merge(rects)
                    .attr("x",function(d){return d;})
    								
    }
    render([100, 300]);
    render([100, 200, 400]);
    //render([100, 400]);
    render([300]);
    render([100, 200, 300,400]);

</script>
</body>

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