default values for keys in a configuration object for a function using goog.object.extend()

after--example_goog.object.extend_config_defaults.js

/**
 * @typedef {{ 
 *       exportFormat: string,
 *       renderFormat: string,
 *       exportArguments: Array }}
 */
some.namespace.MyClass.ARGUMENTS;

/**
 * @param {some.namespace.MyClass.ARGUMENTS=} opt_config optional configuration object
 */
some.namespace.MyClass = function(opt_config) {
  /** @ type {!some.namespace.MyClass.ARGUMENTS} */
  this.config_ = goog.object.extend({
    textContent: 'default',
    tagType: 'text-link',
    applyArguments: []
  }, opt_config);
};

/* neater, easier to read and non-stringy */

before--example_goog.object.extend_config_defaults.js

/**
 * @param {object=} opt_config optional configuration object
 */
some.namespace.MyClass = function(opt_config) {
    /* ... */
    this.config_ = opt_config
    goog.object.setIfUndefined(config_, 'exportFormat', 'default');
    goog.object.setIfUndefined(config_, 'renderFormat', 'text-link');
    goog.object.setIfUndefined(config_, 'exportArguments', []);
    /* ... */
};