Example Download

Generally, when you want to download a file from a web application, you must make a request to the server for the file in question so that it can later be downloaded through the user's browser; with a few lines of code and using data URIs it is possible to create files to be downloaded without the need to make this type of requests; the JavaScript code that we will see below allows you to generate a plain text file (TXT) from the content of a text field; specifically from a textarea so it can later be downloaded; First we will see how to create a data URI:

Data URI Syntax

Data URIs allow content to be embedded in small files; Taking an excerpt from the official documentation offered by MDN on data URIs, they comply with the following syntax:

data:[<mediatype>][;base64],<data>

<mediatype>	Allows you to indicate the file type as: Text: Textual (readable) data text such as text/plain (default) or text/html. Image: Binary data to represent images: image/jpeg, image/gif, image/png. Audio: Digital sound data: audio/basic, audio/wav video. Video: Video data: video/mpeg.
;base64 (optional)	If it is flat or textual data, the text is simply embedded; otherwise you must specify ;base64 to embed binary data such as images or videos.

The next step indicates the date of the file; example:

data:,Hello%2C%20World!

It corresponds to a plain text file.

Script to generate and download files

The HTML

<textarea id="txt"></textarea>
<a href="#" id="link" download="contenido.txt">Descargar el contenido del textarea</a>

The Javascript

    window.onload = function() {
      var txt = document.getElementById('txt');
      txt.value = window.onload + '';
	   document.getElementById('link').onclick = function(code) {
        this.href = 'data:text/plain;charset=utf-8,'
          + encodeURIComponent(txt.value);
      };
    };

How does the script to generate and download files work?

The heart of the script:

     document.getElementById('link').onclick = function(code) {
        this.href = 'data:text/plain;charset=utf-8,'
          + encodeURIComponent(txt.value);
      };

The encodeURIComponent() function encodes the text string and embeds it within the link's href attribute for download through the link's download attribute when a click is detected.

Original article: JAVASCRIPT – CREATING A DOWNLOADABLE FILE IN THE BROWSER.

Example Download