有时候，我们绘制好的canvas想存储为本地图片，该怎么做呢?canvas提供了一个重要的方法toDataURL()，这个方法能把画布里的图案转变成base64编码格式的png或者其他格式的图片(根据你传入的mine类型的参数)，然后返回 Data URL数据。接下来我们看具体是怎么实现的。

html页面一个canvas画布：

<canvas width=200 height=200 id="thecanvas"></canvas>
<button id="saveImageBtn">Save Image</button>
<button id="downloadImageBtn">Download Image</button>

对应的js代码实现：

window.onload = function() {
        draw();
        var saveButton = document.getElementById("saveImageBtn");
        bindButtonEvent(saveButton, "click", saveImageInfo);
        var dlButton = document.getElementById("downloadImageBtn");
        bindButtonEvent(dlButton, "click", saveAsLocalImage);
      };
            function draw(){
                var canvas = document.getElementById("thecanvas");
                var ctx = canvas.getContext("2d");
                ctx.fillStyle = "rgba(125, 46, 138, 0.5)";
                ctx.fillRect(25,25,100,100); 
                ctx.fillStyle = "rgba( 0, 146, 38, 0.5)";
                ctx.fillRect(58, 74, 125, 100);
                ctx.fillStyle = "rgba( 0, 0, 0, 1)"; // black color
                ctx.fillText("Gloomyfish - Demo", 50, 50);
            }
            
            function bindButtonEvent(element, type, handler)
{
                 if(element.addEventListener) {
                    element.addEventListener(type, handler, false);
                 } else {
                    element.attachEvent('on'+type, handler);
                 }
      }
            
            function saveImageInfo () 
{
              var mycanvas = document.getElementById("thecanvas");
              var image    = mycanvas.toDataURL("image/png");
              var w=window.open('about:blank','image from canvas');
              w.document.write("<img src='"+image+"' alt='from canvas'/>");
            }
 
            function saveAsLocalImage () {
              var myCanvas = document.getElementById("thecanvas");
            // here is the most important part because if you dont replace you will get a DOM 18 exception.
            // var image = myCanvas.toDataURL("image/png").replace("image/png", "image/octet-stream;Content-Disposition: attachment;filename=foobar.png");
            var image = myCanvas.toDataURL("image/png").replace("image/png", "image/octet-stream"); 
            window.location.href=image; // it will save locally
          }

点击save按钮后，显示图片，点击图片即可弹出下载对话框。