lib/mixins/link.js
function _buildFunction(resource) {
  return function(callback) {
    if (typeof callback == 'function') {
      if (resource !== null) return resource.exec.call(resource, callback);
      else return callback(null, null);
    }
    return resource;
  }
}
function _buildFunction(resource) {
  return function(callback) {
    if (typeof callback == 'function') {
      if (resource !== null) return resource.exec.call(resource, callback);
      else return callback(null, null);
    }
    return resource;
  }
}
Builds the function for the resource.

@param {Object|Null} resource The resource for the getter.
@return {Function}
@api private