原生javascript 制作canvas 验证码

<canvas id="canvas" width="120" height="40"></canvas>
        <a href="#" id="changeImg">看不清，换一张</a>
        <script>
            /**生成一个随机数**/
            function randomNum(min, max) {
                return Math.floor(Math.random() * (max - min) + min);
            }
            /**生成一个随机色**/
            function randomColor(min, max) {
                var r = randomNum(min, max);
                var g = randomNum(min, max);
                var b = randomNum(min, max);
                return "rgb(" + r + "," + g + "," + b + ")";
            }
            drawPic();
            document.getElementById("changeImg").onclick = function(e) {
                e.preventDefault();
                drawPic();
            }

            /**绘制验证码图片**/
            function drawPic() {
                var canvas = document.getElementById("canvas");
                var width = canvas.width;
                var height = canvas.height;
                var ctx = canvas.getContext(‘2d‘);
                ctx.textBaseline = ‘bottom‘;  //文本对齐方式

                /**绘制背景色**/
                ctx.fillStyle = randomColor(180, 240); //颜色若太深可能导致看不清
                ctx.fillRect(0, 0, width, height);
                /**绘制文字**/
                var str = ‘ABCEFGHIJKLMNOPQRSTUVWXYZ123456789‘;
                for(var i = 0; i < 4; i++) {
                    var txt = str[randomNum(0, str.length)];
                    ctx.fillStyle = randomColor(50, 160); //随机生成字体颜色
                    ctx.font = randomNum(30, 50) + ‘px SimHei‘; //随机生成字体大小
                    var x = 10 + i * 25;
                    var y = randomNum(25, 45);
                    var deg = randomNum(-45, 45);
                    //修改坐标原点和旋转角度
                    ctx.translate(x, y);
                    ctx.rotate(deg * Math.PI / 180);
                    ctx.fillText(txt, 0, 0);
                    //恢复坐标原点和旋转角度
                    ctx.rotate(-deg * Math.PI / 180);
                    ctx.translate(-x, -y);
                }
                /**绘制干扰线**/
                for(var i = 0; i < 8; i++) {
                    ctx.strokeStyle = randomColor(40, 180);
                    ctx.beginPath();
                    ctx.moveTo(randomNum(0, width), randomNum(0, height));
                    ctx.lineTo(randomNum(0, width), randomNum(0, height));
                    ctx.stroke();
                }
                /**绘制干扰点**/
                for(var i = 0; i < 100; i++) {
                    ctx.fillStyle = randomColor(0, 255);
                    ctx.beginPath();
                    ctx.arc(randomNum(0, width), randomNum(0, height), 1, 0, 2 * Math.PI);
                    ctx.fill();
                }
            }
        </script>