How to implement download file function in javascript

JavaScript is a programming language widely used in Web front-end development. It can implement many functions, one of which is the file download function.

In Web development, the file download function is often used to provide users with the ability to download certain resources, such as pictures, documents, attachments, etc. With JavaScript, we can enable users to download these files by clicking a button or link.

Next I will introduce two ways to implement the file download function in JavaScript.

First way: Use native JavaScript to download files

Before using native JavaScript, we need to know how to get the file on the server. This can be achieved by using the XMLHttpRequest object. The sample code is as follows:

function downloadFile(url) {
  // 创建XMLHttpRequest对象
  var xhr = new XMLHttpRequest();
  xhr.open('GET', url, true);
  xhr.responseType = 'blob';

  // 注册事件监听函数
  xhr.onload = function() {
    if (xhr.status === 200) {
      // 成功获取文件
      var blob = xhr.response;
      var fileName = getFileNameFromHeader(xhr.getResponseHeader('Content-Disposition'));

      // 创建a标签，用于下载该文件
      var a = document.createElement('a');
      a.href = window.URL.createObjectURL(blob);
      a.download = fileName;
      a.style.display = 'none';
      document.body.appendChild(a);
      a.click();

      // 释放资源
      window.URL.revokeObjectURL(a.href);
      document.body.removeChild(a);
    } else {
      // 获取文件失败
      console.log('Failed to download file');
    }
  };

  // 发送请求
  xhr.send();
}

function getFileNameFromHeader(header) {
  if (!header) {
    return null;
  }

  var match = header.match(/filename=\"(.+)\"/);
  if (match != null && match.length > 1) {
    return match[1];
  }

  return null;
}

In the above code, we create an XMLHttpRequest object and send a GET request by specifying the URL (that is, the file address on the server). In addition, we also specified the responseType as 'blob', which means that the response body is a Blob object to facilitate our file download processing.

After we successfully obtain the file, we implement the file download function by creating a tag. Before downloading, we need to specify the href attribute of the a tag as the URL of the Blob object, and set the download attribute to specify the file name. Then, by calling the click method of a, the function of automatically downloading the file can be realized.

It should be noted that after the download is completed, we need to release the Blob URL resource to prevent memory leaks.

Second method: Call the FileSaver.js library to download files

In addition to using native JavaScript, we can also use existing third-party libraries to quickly implement the file download function. Here, we recommend using the FileSaver.js library, which provides a series of APIs to quickly implement file downloads and has good cross-browser compatibility.

First, we need to download the library from the official website of FileSaver.js (https://github.com/eligrey/FileSaver.js/) and introduce the library into the HTML page:

<script src="path/to/FileSaver.js"></script>

Then, we can implement the file download function through the following code:

function downloadFile(url, fileName) {
  // 创建XMLHttpRequest对象
  var xhr = new XMLHttpRequest();
  xhr.open('GET', url, true);
  xhr.responseType = 'blob';

  // 注册事件监听函数
  xhr.onload = function() {
    if (xhr.status === 200) {
      // 成功获取文件
      var blob = xhr.response;

      // 使用FileSaver.js库实现文件下载
      window.saveAs(blob, fileName);
    } else {
      // 获取文件失败
      console.log('Failed to download file');
    }
  };

  // 发送请求
  xhr.send();
}

In the above code, we obtain the file on the server by creating an XMLHttpRequest object. After we successfully obtain the file, we can use the saveAs API of FileSaver.js to automatically download the file. It is important to note that we also need to specify the file name.

Summary:

Through native JavaScript or the FileSaver.js library, we can achieve a convenient and fast file download function. Among them, the native JavaScript method is more cumbersome, but it can better understand the implementation principle of file downloading, while the FileSaver.js library provides a more convenient API, which can greatly reduce the amount of code and development time. Either way, the ability to download files can help us better enhance the user experience while meeting user needs.

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