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 length = arguments.length,
          object = arguments[0];

      if (length < 2 || object == null) {
        return object;
      }
      if (length > 3 && isIterateeCall(arguments[1], arguments[2], arguments[3])) {
        length = 2;
      }
      // Juggle arguments.
      if (length > 3 && typeof arguments[length - 2] == 'function') {
        var customizer = bindCallback(arguments[--length - 1], arguments[length--], 5);
      } else if (length > 2 && typeof arguments[length - 1] == 'function') {
        customizer = arguments[--length];
      }
      var index = 0;
      while (++index < length) {
        var source = arguments[index];
        if (source) {
          assigner(object, source, customizer);
        }
      }
      return object;
    };
  }

  return createAssigner;
});