<!DOCTYPE html>
<html>
<head>
   <title>canvas example</title>
    <meta charset="utf-8">
</head>
<body>
<canvas id="canvas" width="600px" height="600px" style="background: deeppink;"></canvas>

<script>
    var canvas=document.getElementById("canvas");
    var context=canvas.getContext('2d');
    const WIDTH=canvas.width;
    const HEIGHT=canvas.height;
    function calculate(){

    var r=Math.floor(Math.random()*256);
    var g=Math.floor(Math.random()*256);
    var b=Math.floor(Math.random()*256);
    var x,y,R;
    while(true){
        x=WIDTH*Math.random();
        y=HEIGHT*Math.random();
        R=Math.min(Math.random()*HEIGHT,Math.random()*WIDTH);
        if(x+R<=WIDTH&&y+R<=HEIGHT&&x>=R&&y>=R){
            break;
        }
    }
    context.beginPath();
    context.strokeStyle="rgb("+r+","+g+","+b+")";
    context.arc(x,y,R,0,Math.PI*2,false);
    context.stroke();
    setTimeout('calculate()',500);
    }
    calculate();

</script>
</body>
</html>
效果图如下所示：