lodash/internal/createAssigner.js

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

  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
  var undefined;

  /**
   * Creates a function that assigns properties of source object(s) to a given
   * destination object.
   *
   * **Note:** This function is used to create `_.assign`, `_.defaults`, and `_.merge`.
   *
   * @private
   * @param {Function} assigner The function to assign values.
   * @returns {Function} Returns the new assigner function.
   */
  function createAssigner(assigner) {
    return restParam(function(object, sources) {
      var index = -1,
          length = object == null ? 0 : sources.length,
          customizer = length > 2 ? sources[length - 2] : undefined,
          guard = length > 2 ? sources[2] : undefined,
          thisArg = length > 1 ? sources[length - 1] : undefined;

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

  return createAssigner;
});