Why must my D3 selection come before the D3 code itself?
Question
I spent hours trying to figure out why my code was not working. I then arbitrarily moved my button code from after the D3 code (at the end between </script> and </body> ) to the top (between <script type="text/javascript"> and <body> ). It works now, but I don't know why. I don't want to make this mistake again or confuse myself in the future.
<body>
<button>Update</button>
<script type="text/javascript">
var w = 500;
var h = 500;
var barPadding = 1;
var dataset = [ ];
for (var i = 0; i < 14; i++) {var newNumber = Math.round(Math.random() * 70);
dataset.push(newNumber);}
//Create SVG element
var svg = d3.select("body")
.append("svg")
.attr("width", w)
.attr("height", h);
//Create Scales for Data conversion
var xScale = d3.scale.linear()
.domain([0, d3.max(dataset, function(d,i) {return d;})]) //input
.range([0,w]); // output
var yScale = d3.scale.ordinal()
.domain(d3.range(dataset.length))
.rangeRoundBands([0, h], 0.05); //Vertical separation + barpadding
svg.selectAll("rect")
.data(dataset)
.enter()
.append("rect")
.attr("x", 3)
.attr("y", function (d,i) {return yScale(i);})
.attr("width", function(d,i) {return xScale(d);})
.attr("height", yScale.rangeBand())
.attr("fill", function(d) {return "rgb(" + (d * 10) + ", 0,0 )";});
svg.selectAll("text")
.data(dataset)
.enter()
.append("text")
.text(function(d) {return d;})
.attr("x", function(d) {return xScale(d) -15;})
.attr("y", function(d, i) {return yScale(i) +5 +yScale.rangeBand() / 2;})
.attr("fill", "white")
.attr("font-family", "sans-serif")
.attr("text-anchor", "middle");
//Create Data Update and transition
d3.select("button")
.on("click", function() {
//New values for dataset
dataset = [ ];
for (var i = 0; i < 14; i++) {var newNumber = Math.round(Math.random() * 70);
dataset.push(newNumber);}
//Update all rects, and color gradient
svg.selectAll("rect")
.data(dataset)
.transition()
.attr("x", 3)
.attr("width", function(d,i) {return xScale(d);})
.attr("fill", function(d) {return "rgb(" + (d * 10) + ", 0,0 )";});
//Update text label and position
svg.selectAll("text")
.data(dataset)
.text(function(d) {return d;})
.attr("x", function(d) {return xScale(d) -15;})
.attr("y", function(d, i) {return yScale(i) +5 + yScale.rangeBand() / 2;});
});
</script>
</body>
1 answers
solution1
0 ACCPTED 2013-10-12 11:02:23
If you're saying that the code as shown in your question works, with the <button> element before the <script> element, it's because <script> elements are executed as the browser encounters them, top-to-bottom while parsing the page. Which means that any JavaScript that you use is only able to reference elements that are higher in the page source because the browser doesn't know about the later elements yet.
Unless you have code within functions that don't get called until after the DOM is complete, for example if you assign a DOM ready or onload event handler, or a delegated click handler or something.
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