defaults.js

var baseRest = require('./_baseRest'),
    eq = require('./eq'),
    isIterateeCall = require('./_isIterateeCall'),
    keysIn = require('./keysIn');

/** Used for built-in method references. */
var objectProto = Object.prototype;

/** Used to check objects for own properties. */
var hasOwnProperty = objectProto.hasOwnProperty;

/**
 * Assigns own and inherited enumerable string keyed properties of source
 * objects to the destination object for all destination properties that
 * resolve to `undefined`. Source objects are applied from left to right.
 * Once a property is set, additional values of the same property are ignored.
 *
 * **Note:** This method mutates `object`.
 *
 * @static
 * @since 0.1.0
 * @memberOf _
 * @category Object
 * @param {Object} object The destination object.
 * @param {...Object} [sources] The source objects.
 * @returns {Object} Returns `object`.
 * @see _.defaultsDeep
 * @example
 *
 * _.defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
 * // => { 'a': 1, 'b': 2 }
 */
var defaults = baseRest(function(object, sources) {
  object = Object(object);

  var index = -1;
  var length = sources.length;
  var guard = length > 2 ? sources[2] : undefined;

  if (guard && isIterateeCall(sources[0], sources[1], guard)) {
    length = 1;
  }

  while (++index < length) {
    var source = sources[index];
    var props = keysIn(source);
    var propsIndex = -1;
    var propsLength = props.length;

    while (++propsIndex < propsLength) {
      var key = props[propsIndex];
      var value = object[key];

      if (value === undefined ||
          (eq(value, objectProto[key]) && !hasOwnProperty.call(object, key))) {
        object[key] = source[key];
      }
    }
  }

  return object;
});

module.exports = defaults;