Nunjuck highlight.js tag extension

highlightjs-tag-nunjucks.js

//
// Allows highlighting of code blocks in Nunjucks template.
// https://mozilla.github.io/nunjucks/api.html#custom-tags
//
const nunjucks = require('nunjucks');
const hljs = require('highlight.js');

function HighlightJsExtension() {
  this.tags = ['highlightjs'];

  this.parse = function (parser, nodes) {
    const tok = parser.nextToken();  // Get the tag token

    // Parse the args and move after the block end.
    const args = parser.parseSignature(null, true);
    parser.advanceAfterBlockEnd(tok.value);

    // Parse the body
    const body = parser.parseUntilBlocks('highlightjs', 'endhighlightjs');
    parser.advanceAfterBlockEnd();

    // Actually do work on block body and arguments
    return new nodes.CallExtension(this, 'run', args, [body]);
  };

  this.run = function (context, language, bodyCallback) {
    const rawCode = bodyCallback();
    const code = hljs.highlightAuto(rawCode, [language]);
    const html = `<pre><code class="hljs language-${language.toLowerCase()}">${code.value}</code></pre>`;
    return new nunjucks.runtime.SafeString(html);
  };
}

const TEMPLATE_DIR = './template/';
const templateLoader = new nunjucks.FileSystemLoader(TEMPLATE_DIR);
const env = new nunjucks.Environment(templateLoader);
env.addExtension('HighlightJsExtension', new HighlightJsExtension());

how to use?