Downloading Data URLs as Files in JavaScript

In modern web development, it’s often necessary to generate files dynamically and allow users to download them. One way to achieve this is by using data URLs, which are URLs that contain the actual file data instead of pointing to a separate resource. In this tutorial, we’ll explore how to download data URLs as files in JavaScript.

Introduction to Data URLs

Data URLs are a type of URL that contains the actual file data, rather than pointing to a separate resource. They’re often used for small files, such as images or icons, and can be embedded directly into HTML or CSS files. The general format of a data URL is:

data:[<mime type>][;charset=<charset>][;base64],<encoded data>

For example:

data:text/plain;base64,SGVsbG8gd29ybGQh

This data URL contains the text "Hello World!" encoded in base64.

Downloading Data URLs as Files

To download a data URL as a file, we can use the download attribute on an <a> element. This attribute specifies the filename that should be used when downloading the linked resource.

Here’s an example of how to download a data URL as a file:

function downloadDataUrl(filename, dataUrl) {
  const link = document.createElement("a");
  link.download = filename;
  link.href = dataUrl;
  document.body.appendChild(link);
  link.click();
  document.body.removeChild(link);
}

const dataUrl = "data:text/plain;base64,SGVsbG8gd29ybGQh";
downloadDataUrl("hello.txt", dataUrl);

This code creates a new <a> element, sets its download attribute to the desired filename, and its href attribute to the data URL. It then appends the link to the document body, simulates a click event on the link, and removes the link from the document body.

Cross-Browser Compatibility

The above code should work in most modern browsers, including Chrome, Firefox, and Edge. However, older versions of Internet Explorer may not support the download attribute.

To achieve cross-browser compatibility, you can use a library like Downloadify, which uses Flash to download files. However, this library has not been updated since 2010 and should not be used for modern web applications.

Alternatively, you can use the fetch API to download the data URL as a blob, and then create a new <a> element with the blob as its href attribute:

function downloadDataUrl(filename, dataUrl) {
  fetch(dataUrl)
    .then(response => response.blob())
    .then(blob => {
      const link = document.createElement("a");
      link.href = URL.createObjectURL(blob);
      link.download = filename;
      link.click();
    })
    .catch(console.error);
}

This code uses the fetch API to download the data URL as a blob, creates a new <a> element with the blob as its href attribute, and simulates a click event on the link.

Conclusion

Downloading data URLs as files in JavaScript can be achieved using the download attribute on an <a> element. While this approach works in most modern browsers, older versions of Internet Explorer may not support it. To achieve cross-browser compatibility, you can use alternative approaches like the fetch API or libraries like Downloadify. However, be sure to consider the security implications and limitations of each approach before implementing them in your application.