canvas in HTML5 and creating multiple circles - html

Question

following the w3c school explanation of canvas, I understand creating shapes...
<script>
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
var radius = 20;
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = 'green';
context.fill();
context.lineWidth = 5;
context.strokeStyle = '#003300';
context.stroke();
</script>
This will make a green circle where my canvas is.
<canvas id="myCanvas"></canvas>
However - I want to apply that "circle" to multiple places on the page, and doing this by ID would be ridiculous.
How does one apply a context (as defined above) to multiple canvas'?? I' assume using class, this seems the logical way.
<canvas class="redDot"></canvas>
<canvas class="redDot"></canvas>
<canvas class="redDot"></canvas>
<canvas class="greenDot"></canvas>
<canvas class="greenDot"></canvas>
<canvas class="blueDot"></canvas>
<canvas class="blueDot"></canvas>
<canvas class="blueDot"></canvas>

Answer 1

You would have to iterate through each Element to apply the changes. I.E:
var dots = document.getElementsByClassName('dots');
for (var i=0;i<dots.length;i++){
var canvas = dots[i];
var context = canvas.getContext('2d');
// Draw circles here
}
Ideally, you would only have one canvas element which you can draw multiple circles
var canvas = document.getElementById('myOneAndOnlyCanvas');
// Using CSS and layering can make background
var context = canvas.getContext('2d');
dot('red',50,50);
dot('blue',100,50);
//etc..
function dot(color,x,y){
context.beginPath();
context.arc(y, y, radius, 0, 2 * Math.PI, false);
context.fillStyle = color;
context.fill();
context.lineWidth = 5;
context.strokeStyle = '#003300';
context.stroke();
}
But if that doesn't fit your use case what about using an SVG object?
Since you mentioned w3 schools: http://www.w3schools.com/svg/
But ideally check out: http://www.w3.org/Graphics/SVG/
Mind, Dom Heavy pages can seriously hurt load times. Depending on what you want to do it might be wiser just to use an image (eg. a large cluster of dots).