node-alt-compare/gulp/gulpfile.js

// For using gulp
const gulp = require('gulp');
// For reading template parts
const fs = require("fs");
// For parsing codes, variables, loops etc. inside html templates
const nunjucks = require('gulp-nunjucks');
// For allowing custom loader
const nunj = require('nunjucks');
// For cleaning previous output
//const del = require('del');
const path = require('path');

// Output directory
const source_dir='src';
const template_dir='src/html';
const output_dir='public';

var jsonInfo = JSON.parse(fs.readFileSync('src/info.json', 'utf8'));
if ( jsonInfo.languages ) {
  jsonInfo.languages.forEach(function(lang) {
    if ( jsonInfo.default_langs.includes(lang.slug) ) {
      lang.default = 1
    }
  });
}

// Loader to include from relative path
// @source https://github.com/mozilla/nunjucks/issues/1214#issuecomment-579534919
class MyCustomLoader {
  constructor() {
    this.async = true
  }

  isRelative (filename) {
    console.log ('isRelative', filename)
    return true
  }

  resolve(parentName, filename) {
    console.log('resolve', parentName, filename)
    return filename
  }

  getSource (filePath, callback) {
    //console.log('getSource', filePath)
    try {
      const src = fs.readFileSync(filePath, 'utf8')
      const file = {
        src,
        path: filePath
      }
      callback(null, file)
    } catch (error) {
      callback(error)
    }
  }
}

// Runs when someone runs "gulp" on this directory
gulp.task('default', async function(){
  var nunjucksenv = new nunj.Environment([new nunj.FileSystemLoader('views'),
                           new MyCustomLoader()]);
  gulp.src(template_dir+'/index.html')
    .pipe(nunjucks.compile(
      {jsoninfo: jsonInfo}
      , {env: nunjucksenv}
    ))
    .pipe(gulp.dest('public'));
});