html5 fileReader - 如何只读取文件的前N个字符？ [英] html5 fileReader -- how to only read the first N characters of a file?

问题描述

目前我使用如下图案来读取一系列文件的前3个字符：

Currently I use a pattern like the following to read the first 3 characters of a series of files:

var files = e.dataTransfer.files;
for (var i = 0, f; f = files[i]; i++) {
  var fr = new FileReader();
  fr.onload = function(e) { 
    var first_three_chars = e.target.result.substr(0,3);
  }
  fr.readAsText(f);
}

问题是我只对前3个字符感兴趣文件，而这个方法读取整个文件，浪费了大量的内存和时间。如何快速迭代文件，只需快速查看第一个字符？

The trouble is that I'm only interested in the first 3 characters of the file, whereas this method reads the entire file, wasting lots of memory and time. How can I quickly iterate over the files, simply taking quick peeks at the first characters?

编辑：slice（）就是答案，谢谢sshen。我是这样做的：

slice() was the answer, thanks sshen. Here's how I did it:

var files = e.dataTransfer.files;
for (var i = 0, f; f = files[i]; i++) {
  var fr = new FileReader();
   fr.onloadend = function(e) {
    if (e.target.readyState == FileReader.DONE) {
      var first_three_chars = e.target.result;
    }
  };
  var blob = f.slice(0, 3);
  fr.readAsText(blob);
}

推荐答案

您可以使用 .slice 方法。您可以阅读更多此处

You can use the .slice method. You can read more here

var reader = new FileReader();

reader.onloadend = function(evt) 
{
    if (evt.target.readyState == FileReader.DONE)  // DONE == 2
    {
        alert(evt.target.result);
    }
};

var blob = file.slice(start, stop + 1);
reader.readAsBinaryString(blob);

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