Simple nodejs template engine

template-engine.js

function template(source, options = {}) {
  const testers = {
    replacer: /#\{\s?[\w]+\s?(\|\|\s?(\'|\")[\w]+(\'|\")\s?)?\}|\{\s?\{\s?[\w]+\s?(\|\|\s?(\'|\")[\w]+(\'|\")\s?)?\}\s?\}/gi,
    alternative: /(\'|\")(.*)(\'|\")/i,
    key: /#\{\s?([\w]+)\s?(\|\|\s?(\'|\")[\w]+(\'|\")\s?)?\}|\{\s?\{\s?([\w]+)\s?(\|\|\s?(\'|\")[\w]+(\'|\")\s?)?\}\s?\}/i,
    doubleBrackets: /\{\{.*\}\}/g
  };

  return source.split("\n").map(row => {
    const isRepleaceable = testers.replacer.test(row);
    const isDoubleBrackets = testers.doubleBrackets.test(row);
    let key = null;
    let alternative = null;

    if (isRepleaceable) {
			key = row.match(testers.key)[1];
			if ( testers.alternative.test(row) ) {
				alternative = row.match(testers.alternative)[2];
			}

      if (isDoubleBrackets) {
        key = row.match(testers.key)[5];
      }

      if ( options[key] ) {
        return row.replace(testers.replacer, options[key])
      } else if ( alternative ) {
				return row.replace(testers.replacer, alternative)        
			}

			return row;
    }

    return row;
  }).join('\n');
}