js压缩上传图片base64长度

im发送图片，现将图片压缩再上传

1) 调用 FileReader 的 reader.readAsDataURL(img); 方法, 在其onload事件中, 将用户选择的图片读入 Image对象.

2) 在image对象的 onload 事件中, 通过 canvas 的 canvas.getContext('2d') 的 drawImage 方法, 将Image 改变大小绘制到canvas上.

3) 通过 canvas.toDataURL("image/jpeg", 1); 方法, 将图片变成base64字符串, 放到send_image_value

toDataURL

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1	canvas.toDataURL(type, encoderOptions);

返回值

　　包含 data URI 的DOMString

type

　　图片格式，默认为 image/png

encoderOptions

　　在指定图片格式为 image/jpeg 或 image/webp的情况下，可以从 0 到 1 的区间内选择图片的质量。如果超出取值范围，将会使用默认值 0.92。其他参数会被忽略。

html

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<input id="sendImage" title="send a picture"  type="file" accept="image/*" onchange="imgChange(this)">   <input type="hidden" value="" id="send_image_value">   <img id="showLoadingimg" src="'+CHAT_SITE_URL+'/templates/default/images/loading.gif" style="position:relative;left:200px;top:200px;z-index:999;display:none;">

　　

js

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function imgChange(e){     //检查是否有文件被选中         if (e.files.length != 0) {             var file = e.files[0],             fileType = file.type,             reader = new FileReader();             if (!reader) {                 e.value = '';                 return;             };             var size = file.size;             var max_size = 2*1024*1024;             if(size>max_size){                 e.value = '';                 $("#send_alert").html('file is too large(>2M)');                 return;             }             $("#showLoadingimg").show();             reader.onload = function(e) {                 //读取成功，显示到页面并发送到服务器                 e.value = '';                 var org_img = e.target.result;                 var image_base64 = org_img;                 if(size>1024*80){//>80K的                     var img = new Image();                      img.src = org_img;                      img.onload=function(){                         var scale = 1;                            if(this.width > 300 || this.height > 300){                                  if(this.width > this.height){                                    scale = 300 / this.width;                              }else{                                    scale = 300 / this.height;                                }                            }                          var canvas = document.createElement("canvas"), drawer = canvas.getContext("2d");                         if(scale!=1) {//按最大高度等比缩放                             img.width  *= scale;                             img.height *= scale;                         }                         canvas.width = img.width;                         canvas.height = img.width * (img.height / img.width);                         drawer.drawImage(img, 0, 0, canvas.width, canvas.height);                         var tmp_code = image_base64 = canvas.toDataURL(fileType);                         if(tmp_code!='data:,'){                             image_base64 = tmp_code;                         }                         img_send(image_base64);                     };                                            }else{                     img_send(image_base64);                 }                               };             reader.readAsDataURL(file);         }     }     /**      *为将图片赋值给消息     **/     function img_send(image_base64){         if(image_base64!='data:,'){             $("#send_image_value").val(image_base64);             send_msg();             $("#showLoadingimg").hide();             $('#sendImage').val("");         }     }

开始时，toDataURL获取的值是data:,

$("#send_image_value").val(image_base64);
send_msg();

这两句写在

reader.onload方法的最下面这就导致图片并没有压缩

因为img.onload还没执行完

发送的还是原来的图片

在调整后就可以实现图片的压缩了