图片上传测量的闭包问题

Question

要在上传时检查上传的 img 的原始宽度和高度（然后相应地调整其大小），我使用以下方法：

var reader = new FileReader();

//Initiate the JavaScript Image object.
var image = new Image(0,0);

//Read the contents of Image File.
reader.readAsDataURL(fileUpload.files[0]);
reader.onload = function () {

  //Set the Base64 string return from FileReader as source.
  image.src = this.result;

  //Validate the File Height and Width.
  image.onload = function () {

    var height = this.naturalHeight;
    var width = this.naturalWidth;
    if (height > 100 || width > 100) {

      // Do needed transformation      

    }
    
  };
};

好的，到目前为止一切顺利。但是现在，当我将其更改为多个文件上传的迭代时，如下所示：

for ( var i = 0; i < fileUpload.files.length; i++ ) {

  var reader = new FileReader();

  //Initiate the JavaScript Image object.
  var image = new Image(0,0);

  var iteratedFile = fileUpload.files[i];

  //Read the contents of Image File.
  reader.readAsDataURL(iteratedFile);
  reader.onload = function () {

    //Set the Base64 string return from FileReader as source.
    image.src = this.result;

    //Validate the File Height and Width.
    image.onload = function () {

      var height = this.naturalHeight;
      var width = this.naturalWidth;
      if (height > 100 || width > 100) {

        // Fails to do anything related to i; i is undefined here, returns undefined too 
        console.log( fileUpload.files[i] );

        // Returns the last iterated name
        console.log( iteratedFile );    

      }
    
    };
  };

}

我无法访问内部侦听器中的迭代文件。怎么会？当我在我写的地方写 console.log( iteratedFile ) 时，我实际上总是得到最后一个迭代文件的名称，这让我注意到这是一个闭包问题。但是，当我在同一个地方而不是那一行写 console.log( i ); 或 console.log(fileUpload.files[i]) 时，我得到 undefined，我不太明白。我怎样才能使 i / 迭代文件在最内部范围内可用（以访问其名称、其 mime 等内容）？

更新

通过在reader.onload function的函数定义末尾添加(i)；我现在可以 console.log 外部函数内部的 i 的正确值，用于所有三个迭代，但随后收到 GET 请求 404 错误发送到控制台中显示的 pages_URL/undefined。 WTH?!

Answer 1

将 i 的词法环境限定为在循环内调用的 IIFE 实际上足以让它像魅力一样工作：

for ( var i = 0; i < fileUpload.files.length; i++ ) {

  (function(){

    var reader = new FileReader();

    //Initiate the JavaScript Image object.
    var image = new Image(0,0);

    var iteratedFile = fileUpload.files[i];

    //Read the contents of Image File.
    reader.readAsDataURL(iteratedFile);
    reader.onload = function () {

      //Set the Base64 string return from FileReader as source.
      image.src = this.result;

      //Validate the File Height and Width.
      image.onload = function () {

        var height = this.naturalHeight;
        var width = this.naturalWidth;
        if (height > 100 || width > 100) {

          // Works now
          console.log( fileUpload.files[i] );

          // Returns the iterated file
          console.log( iteratedFile );    

        }
    
      };
    };

  }());

}

图片上传测量的闭包问题

Closure problem with image upload measurement

javascript

upload

closures

file

onload