D3 Examples - Foundations of D3: Data Binding Scope

A Lesson in Binding Data

In an effort incrementally introduce complexity, data is bound one datum at a time with a for loop to blend the declarative/functional paradigm of D3 and the imperative/procedural paradigm of standard Javascript.

In this example we can see how D3's data binding functions (data() and datum()) create a closure encapsulating the state of variables at the time the data is bound.

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
    svg { width: 100%; height: 100%; }
    circle {
      opacity: 0.5;
        ;
    }
       
  </style>
</head>
​
<body> 
  <svg>
    </svg>
​
  <script>
    var width = 960;
    var height = 500;
​
    var svg = d3.select('svg');
    var colors = ["grey", "yellow"]
    
    for(var i = 0; i < 1800; i++) {
      var data = {
        x: Math.random() * width,
        y: Math.random() * height,
        radius: 2,
        color: colors[i%colors.length]
      };
      
      // each cicle gets a different span value
      svg.append('circle')
      
      // datum bind a closure of the current variables.
            .datum(data);
    }
    
    // new value for width doesn't affect bound data
    width = 2000000;
    
    // the last value from the for loop
    console.log(data);
    
    // mutate the circles' visual appearance
    svg.selectAll('circle')
        .attr('cx', function(d) {
            console.log(d);
            return d.x;
            })
        .attr('cy', function(d) {
            return d.y;
            })
            .attr('fill', function(d) {
            return d.color;
            })
        .attr('r', function(d) {
            return d.radius;
            })
    
    console.log(d3.selectAll('circle'));
  </script>
</body>
​
    