如何在网上实现文件上传进度条？

在网上实现文件上传进度条，可以通过以下几个步骤来实现：

使用HTML和JavaScript创建一个简单的表单和进度条。

HTML代码示例：

<form id="uploadForm">
 <input type="file" id="fileInput" name="file" />
 <button type="submit">上传文件</button>
</form>
<div id="progressBarContainer">
  <div id="progressBar"></div>
</div>

JavaScript代码示例：

const uploadForm = document.getElementById("uploadForm");
const fileInput = document.getElementById("fileInput");
const progressBar = document.getElementById("progressBar");

uploadForm.addEventListener("submit", (event) => {
  event.preventDefault();
  const file = fileInput.files[0];
  const formData = new FormData();
  formData.append("file", file);

  const xhr = new XMLHttpRequest();
  xhr.upload.addEventListener("progress", (event) => {
    if (event.lengthComputable) {
      const progress = (event.loaded / event.total) * 100;
      progressBar.style.width = progress + "%";
    }
  });

  xhr.onreadystatechange = function () {
    if (xhr.readyState === 4 && xhr.status === 200) {
      console.log("文件上传成功");
    }
  };

  xhr.open("POST", "https://your-server-url.com/upload");
  xhr.send(formData);
});

在服务器端，创建一个接收文件上传请求的API。

以下是一个使用Node.js和Express框架的示例：

const express = require("express");
const multer = require("multer");

const app = express();
const upload = multer({ dest: "uploads/" });

app.post("/upload", upload.single("file"), (req, res) => {
  res.send("文件上传成功");
});

app.listen(3000, () => {
  console.log("服务器已启动");
});