code: def javascript_link(src_url, src_path=None, cache_bust=None, content='', extra_attrs=None):
      
      if cache_bust:
          append_suffix = get_cache_buster(src_path=src_path, method=cache_bust)
          delim = '&' if '?' in src_url else '?'
          src_url += delim + append_suffix
  
      attrs = {
          'src': src_url,
          'type': 'text/javascript',
      }
      if extra_attrs:
          attrs.update(extra_attrs)
  
      return tag('script', content=content, attrs=attrs)
docs: Helper for programmatically building HTML JavaScript source include
      links, with optional cache busting.
  
      :param src_url:
          Goes into the `src` attribute of the `<script src="...">` tag.
  
      :param src_path:
          Optional filesystem path to the source file, used when `cache_bust` is
          enabled.
  
      :param content:
          Optional content of the DOM element. If `None`, then the element is
          self-closed.
  
      :param cache_bust:
          Optional method to use for cache busting. Can be one of: importtime,
          md5, or mtime. If the value is md5 or mtime, then `src_path` must be
          supplied.
  
  
      Example::
  
          >>> javascript_link('/static/js/core.js')
          u'<script src="/static/js/core.js" type="text/javascript"></script>'