code: function defaultResolveFn(source, args, context, { fieldName }) {
    // ensure source is a value for which property access is acceptable.
    if (typeof source === 'object' || typeof source === 'function') {
      const property = source[fieldName];
      if (typeof property === 'function') {
        return source[fieldName](args, context);
      }
      return property;
    }
    return undefined;
  }
docs: Copied from https://github.com/graphql/graphql-js/blob/v0.7.1/src/execution/execute.js#L1004 with 'return undefined' added for clarity (and eslint)