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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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forked from margeigh's block: Foundations of D3: Data Binding Scope

forked from margeigh's block: Foundations of D3: Data Binding Scope

<!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;
      fill: yellow
        
        ;
    }
       
  </style>
</head>

<body> 
  <svg>
	</svg>

<script>
    var width = 960;
    var height = 500;

var svg = d3.select('svg');
    
    for(var i = 0; i < 80; i++) {
      span = Math.random() * width;
      
      var data = {
        x: Math.random() * span,
      	y: Math.random() * height,
      	radius: 2,
        span: span
      };
      
      // 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.select('circle')
      	.attr('cx', function(jon) {
      		console.log(jon);
      		return jon.x;
    		})
      	.attr('cy', function(d) {
      		return d.y;
    		})
      	.attr('r', function(d) {
      		return d.radius;
    		})
    
    console.log(d3.selectAll('circle'));
  </script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.5/d3.min.js