How can the fileupload control obtain the content of the uploaded file?

In HTML, using the fileupload control to achieve file upload functionality typically requires using JavaScript to access the content of the uploaded file. Here is a common method:

1. Define the file upload control in HTML.

<input type="file" id="myfileupload" />

1. Retrieve the content of an uploaded file in JavaScript.

var fileUpload = document.getElementById("myfileupload");
var file = fileUpload.files[0]; // 获取上传的第一个文件
var reader = new FileReader();

reader.onload = function(e) {
  var fileContent = reader.result; // 获取文件内容
  console.log(fileContent);
};

reader.readAsText(file); // 以文本格式读取文件内容

In the above code, we first use the getElementById() method to get the fileupload control, then use the files property to obtain the list of uploaded files, and then use the FileReader object to read the file contents. By setting the onload event handler for the FileReader object, we can retrieve the file contents after the file is loaded. In this example, we use the readAsText() method to read the file content in text format. If other file formats need to be read, other related methods such as readAsDataURL() or readAsArrayBuffer() can be used.

Please note that for security reasons, browsers restrict access to uploaded files. Therefore, in order to access the content of an uploaded file in JavaScript, an event must be triggered after the user selects the file, such as clicking a button or submitting a form, otherwise the file content cannot be directly accessed.