JavaScript uses FileReader to achieve image upload preview effect

This article mainly introduces in detail JavaScriptUsing FileReader to achieve image upload preview effect, which has certain reference value for learning JavaScript. Friends who are interested in JavaScript can refer to this article

FileReader is part of the HTML5 File API. It implements an asynchronous file reading mechanism. You can think of FileReader as XMLHttpRequest, the only difference is that it reads the file system instead of the remote server. In order to read the data in the file, FileReader provides the following methods.

• readAsText(file,encoding): Read the file in plain text and save the read file to the result attribute.

• readAsDataURL(file): Read the file and save the file in the result attribute in the form of data URI.

• readAsBinaryString(file): Read the file and save a string in the result attribute. Each character in the string represents one byte.

• readAsArrayBuffer(file): Read the file and save an ArrayBuffer containing the contents of the file in the result attribute.

• multiple attribute indicates support for multiple images

  
  

<p id="wrapper">    
 <input id="fileUpload" type="file" multiple /><br />
 <p id="image-holder"> </p>
</p>

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$("#fileUpload").on('change', function () {
 
  //获取上传文件的数量
  var countFiles = $(this)[0].files.length;
 
  var imgPath = $(this)[0].value;
  var extn = imgPath.substring(imgPath.lastIndexOf('.') + 1).toLowerCase();
  var image_holder = $("#image-holder");
  image_holder.empty();
 
  if (extn == "gif" || extn == "png" || extn == "jpg" || extn == "jpeg") {
    if (typeof (FileReader) != "undefined") {
 
      // 循环所有要上传的图片
      for (var i = 0; i < countFiles; i++) {
 
        var reader = new FileReader();
        reader.onload = function (e) {
          $("<img />", {
            "src": e.target.result,
              "class": "thumb-image"
          }).appendTo(image_holder);
        }
 
        image_holder.show();
        reader.readAsDataURL($(this)[0].files[i]);
      }
 
    } else {
      alert("你的浏览器不支持FileReader！");
    }
  } else {
    alert("请选择图像文件。");
  }
});

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