function highlight(strings, ...values) {
  return strings.reduce((result, str, i) => {
    const value = values[i - 1];
    const formatted =
      typeof value === "number"
        ? `[${value * 2}]`
        : `<${String(value).toUpperCase()}>`;
    return result + formatted + str;
  });
}

const language = "javascript";
const year = 2015;
const feature = "templates";

const output = highlight`Language: ${language}, introduced in ${year}, feature: ${feature}!`;
console.log(output);