Javascript 承诺与 FileReader()

问题描述

我有以下 HTML 代码:

I have the following HTML Code:

<input type='file' multiple>

这是我的 JS 代码:

And Here's my JS Code:

var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = function(){
    var fr = new FileReader();
    for(var i = 0; i < inputFiles.files.length; i++){
        fr.onload = function(){
            console.log(i) // Prints "0, 3, 2, 1" in case of 4 chosen files
        }
    }
    fr.readAsDataURL(inputFiles.files[i]);
}

所以我的问题是，我怎样才能使这个循环同步?那就是首先等待文件完成加载，然后移动到下一个文件.有人告诉我使用 JS Promises.但我无法让它工作.这是我正在尝试的:

So my question is, how can I make this loop synchronous ? That is first wait for the file to finish loading then move on to the next file. Someone told me to use JS Promises. But I can't make it to work. Here's what I'm trying:

var inputFiles = document.getElementsByTagName("input")[0];
inputFiles.onchange = function(){
    for(var i = 0; i < inputFiles.files.length; i++){
        var fr = new FileReader();
        var test = new Promise(function(resolve, reject){
            console.log(i) // Prints 0, 1, 2, 3 just as expected
            resolve(fr.readAsDataURL(inputFiles.files[i]));
        });
        test.then(function(){
            fr.onload = function(){
                console.log(i); // Prints only 3
            }
        });
    };
}

提前致谢...

推荐答案

我们修改了 midos answer 以使其工作以下内容:

We modified midos answer to get it to work the following:

function readFile(file){
  return new Promise((resolve, reject) => {
    var fr = new FileReader();
    fr.onload = () => {
      resolve(fr.result )
    };
    fr.onerror = reject;
    fr.readAsText(file.blob);
  });
}

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