code: function (key) {
      // Assume the middleware is a function
      var middleware = key;
  
      // If the middleware is a string
      if (typeof middleware === 'string') {
        // Look it up and assert it was found
        middleware = MIDDLEWARES[key];
        assert(middleware, 'value-mapper middleware "' + key + '" could not be found."');
      }
  
      // Add the middleware to our stack
      this.middlewares.push(middleware);
    }
docs: Add middleware to instance