How to implement file upload in javascript

With the development of the Internet, file uploading has become a basic requirement for people to use websites and applications. Whether uploading images, videos, documents, or other types of files, JavaScript is a very useful tool that can help developers easily implement file upload functions.

This article will introduce how to use JavaScript to implement the file upload function. We'll discuss the process of selecting a file, validating it, uploading to the server, and how to handle situations when the upload completes or fails.

Select files

The first step to upload files is to select the files to be uploaded. The file selection function can be easily implemented using HTML5's <input type="file"> tag. This tab will pop up a file selection box where the user can select files to upload from the local file system.

<input type="file" id="fileInput">

In JavaScript, we can get the file name of the file selected by the user by getting the value attribute of the input element.

const fileInput = document.querySelector('#fileInput');
const fileName = fileInput.value;

Verify file

After selecting the file, we need to verify whether the file meets our requirements. We can verify file types and sizes to ensure they meet our requirements.

Some file types can be restricted on the front end through the accept attribute of HTML5, for example:

<input type="file" id="fileInput" accept=".jpg,.png,.gif">

This will limit the user to only select .jpg, .png, .gif document.

File size limit The file size can be calculated via JavaScript and compared to the limit value, for example:

const fileInput = document.querySelector('#fileInput');
const file = fileInput.files[0];
const maxSize = 10 * 1024 * 1024; // 10MB

if (file.size > maxSize) {
  alert('文件太大了');
  return;
}

Uploading files

Now we have selected and verified the file to upload , the next step is to upload it. We can use Ajax technology to send file data to the server.

const fileInput = document.querySelector('#fileInput');
const file = fileInput.files[0];
const formData = new FormData();
formData.append('file', file);

const xhr = new XMLHttpRequest();
xhr.open('POST', '/upload');
xhr.send(formData);

In the above code, we use the FormData object to wrap the file data into a form, and then use the XMLHttpRequest object to upload the form to the server. The server can process uploaded files as needed, such as saving to the file system or storing in a database.

Handling upload completion or failure situations

Finally, we need to handle upload completion or failure situations to tell the user that the file was successfully or unsuccessfully uploaded.

const xhr = new XMLHttpRequest();
xhr.open('POST', '/upload');

xhr.addEventListener('load', () => {
  if (xhr.status >= 200 && xhr.status < 300) {
    alert('上传成功');
  } else {
    alert('上传失败');
  }
});

xhr.send(formData);

In the above code, we added an event listener to monitor the status of the upload process. If the value of xhr.status is between 200 and 299, the upload is successful, otherwise the upload fails.

Summary

JavaScript is a useful tool for file uploading. Use the <input type="file"> tag to select the file, use JavaScript to validate the file and send the file data to the server, and then handle the completion or failure of the upload. These simple steps can help you implement file upload functionality with ease.

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