lib/input-generators/_base.js
function supplyMissingDefault (options, name) {
  if (options[name]() === void 0) {
    options[name] = _.wrap(options[name], function(func, key) {
      var res = func(key);
      return res === void 0 ? defaults[name] : res;
    });
  }
}
function supplyMissingDefault (options, name) {
  if (options[name]() === void 0) {
    options[name] = _.wrap(options[name], function(func, key) {
      var res = func(key);
      return res === void 0 ? defaults[name] : res;
    });
  }
}
If a normalized option was overriden, but a default still undefined, supply one.
The result could still be undefined, but only if the default is undefined.

If calling the normalized option with no args is undefined, supply missing default.

@param {Object} options - Input generator options.
@param {String} name - The name of the property to supply missing default for.