def runtime
      @runtime ||= begin
        if ExecJS.eval("typeof Uint8Array") != "function"
          if ExecJS.runtime.name.start_with?("therubyracer")
            raise "ExecJS::RubyRacerRuntime is not supported. " \
              "Please replace therubyracer with mini_racer " \
              "in your Gemfile or use Node.js as ExecJS runtime."
          else
            raise "#{ExecJS.runtime.name} runtime does’t support ES6. " \
              "Please update or replace your current ExecJS runtime."
          end
        end

        if ExecJS.runtime == ExecJS::Runtimes::Node
          version = ExecJS.runtime.eval("process.version")
          first = version.match(/^v(\d+)/)[1].to_i
          if first < 6
            raise "Autoprefixer doesn’t support Node #{version}. Update it."
          end
        end

        ExecJS.compile(build_js)
      end
    end
def runtime
      @runtime ||= begin
        if ExecJS.eval("typeof Uint8Array") != "function"
          if ExecJS.runtime.name.start_with?("therubyracer")
            raise "ExecJS::RubyRacerRuntime is not supported. " \
              "Please replace therubyracer with mini_racer " \
              "in your Gemfile or use Node.js as ExecJS runtime."
          else
            raise "#{ExecJS.runtime.name} runtime does’t support ES6. " \
              "Please update or replace your current ExecJS runtime."
          end
        end

        if ExecJS.runtime == ExecJS::Runtimes::Node
          version = ExecJS.runtime.eval("process.version")
          first = version.match(/^v(\d+)/)[1].to_i
          if first < 6
            raise "Autoprefixer doesn’t support Node #{version}. Update it."
          end
        end

        ExecJS.compile(build_js)
      end
    end
Lazy load for JS library