Reusable Click Handler Technique

<!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; }
    #clickme{
      border:1px solid black;
      padding:15px;
      cursor:pointer;      
    }
    #clickme:hover{
      background:gray;
    }
    
  </style>
</head>
<h1>You clicked on node <span><u id="selected">Selected Node</u></span></h1>
<svg>
</svg>
<h1 id="clickme">Click here to select 'A' programatically</h1>
<body>
  <script>

svg.selectAll("circle")
      .data(nodes)
      .enter()
      .append("circle")
      .attr("r",15)
      .style("fill","white")
      .style("stroke","black")
      .attr("cy",function(d){return d.y})
      .attr("cx",function(d){return d.x})
   		.on("click",function(d,i){
          //rather than calling handleClick directly, which passes through the d3 default (d,i,nodes) 
          //use the .call() functionality which passes through the d3 selection in place of (d,i,nodes)
          //this allows the same .call() to be used whether the function is called on a mouseover or if called programatically. 
          d3.select(this).call(handleClick)
    	})
    	.style("cursor","pointer")
    
    svg.selectAll("text")
      .data(nodes)
      .enter()
      .append("text")
      .style("fill","black")
      .attr("y",function(d){return d.y})
      .attr("x",function(d){return d.x})
			.text(function(d){return d.node})
    	.style("text-anchor","middle")
    	.style("dominant-baseline","middle")
    	.style("pointer-events","none")
    
    function handleClick(selection){
      	var d = selection.data()[0];
      	d3.select("#selected").text(d.node)
        svg.selectAll("circle").style("fill","white")
        selection.style("fill","red")    
    }
    
    d3.select("#clickme")
    	.on("click",function(){
      		//go find the desired node, then pass that selection to the same update function
      		var selection = d3.selectAll("circle").filter(function(d){return d.node =="A"})
          handleClick(selection)
	    })
    
  </script>
</body>

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