toDataURL() - Turns a string that can represent a text document and returns the corresponding data URL (AKA data URI).

toDataURL.js

/**
 * Turns a string that can represent a text document and returns the
 * corresponding data URL (AKA data URI).
 * @param {string} text
 *   The text to turn into a data URL.
 * @param {Object} options
 *   Optional.  An object containing the different options to set.
 * @param {boolean=} options.base64
 *   Optional, defaults to the `false`.  Indicates if the returned data URL
 *   should be base64 encoded.
 * @param {string=} options.charset
 *   Optional.  Indicates the character set of the content.  Examples are
 *   "US-ASCII", "UTF-8", etc.
 * @param {string=} options.type
 *   Optional, defaults to the empty string.  The content type of `text` (eg.
 *   `"text/html"`).
 * @returns {string}
 *   A data URL which represents `text` as the given `type`.
 */
function toDataURL(text, options) {
  let {base64, charset, type} = Object(options);
  return ('data:'
      + (type ?? '')
      + ';'
      + (charset ? 'charset=' + charset + ';' : '')
      + (base64 ? 'base64;' : '')
    ).replace(/;$/, '')
    + ','
    + (base64
      // unescape() and encodeURIComponent() used based on this solution:
      // https://stackoverflow.com/a/26603875/657132
      ? window.btoa(unescape(encodeURIComponent(text)))
      : encodeURIComponent(text)
    );
}

~toAsyncDataURL.js

/**
 * Turns a string that can represent a text document and returns the
 * corresponding data URL (AKA data URI).  Leverages `FileReader` and `Blob`
 * to ensure validity of data URL's contents.
 * @param {string} text
 *   The text to turn into a data URL.
 * @param {Object} options
 *   Optional.  An object containing the different options to set.
 * @param {boolean=} options.base64
 *   Optional, defaults to the `false`.  Indicates if the returned data URL
 *   should be base64 encoded.
 * @param {string=} options.charset
 *   Optional.  Indicates the character set of the content.  Examples are
 *   "US-ASCII", "UTF-8", etc.
 * @param {string=} options.type
 *   Optional, defaults to the empty string.  The content type of `text` (eg.
 *   `"text/html"`).
 * @returns {Promise<string>}
 *   A data URL which represents `text` as the given `type`.
 */
function toAsyncDataURL(text, options) {
  let {base64, charset, type} = Object(options);
  return new Promise(resolve => {
    Object.assign(new FileReader(), {
      onload({target: {result}}) {
        result = /,([^]*$)/.exec(result)[1];
        resolve(
          ('data:'
            + (type ?? '')
            + ';'
            + (charset ? 'charset=' + charset + ';' : '')
            + (base64 ? 'base64;' : '')
          ).replace(/;$/, '')
          + ','
          + (base64 ? result : encodeURIComponent(decodeURIComponent(escape(atob(result)))))
        );
      }
    }).readAsDataURL(new Blob([text], {type: charset ? ';charset=' + charset : ''}));
  });
}