lodash/internal/createAssigner.js

define(['./bindCallback', './isIterateeCall'], function(bindCallback, isIterateeCall) {

  /**
   * Creates a function that assigns properties of source object(s) to a given
   * destination object.
   *
   * @private
   * @param {Function} assigner The function to assign values.
   * @returns {Function} Returns the new assigner function.
   */
  function createAssigner(assigner) {
    return function() {
      var args = arguments,
          length = args.length,
          object = args[0];

      if (length < 2 || object == null) {
        return object;
      }
      var customizer = args[length - 2],
          thisArg = args[length - 1],
          guard = args[3];

      if (length > 3 && typeof customizer == 'function') {
        customizer = bindCallback(customizer, thisArg, 5);
        length -= 2;
      } else {
        customizer = (length > 2 && typeof thisArg == 'function') ? thisArg : null;
        length -= (customizer ? 1 : 0);
      }
      if (guard && isIterateeCall(args[1], args[2], guard)) {
        customizer = length == 3 ? null : customizer;
        length = 2;
      }
      var index = 0;
      while (++index < length) {
        var source = args[index];
        if (source) {
          assigner(object, source, customizer);
        }
      }
      return object;
    };
  }

  return createAssigner;
});