src/config/defaultsDeep.js

'use strict';

module.exports = function defaultsDeep(target, objects) {
  target = target || {};

  function copy(target, current) {
    for (var key in current) {
      if (current.hasOwnProperty(key)) {
        var value = current[key]
        if (key === '__proto__') {
          continue;
        }
        
        var val = target[key];
        // add the missing property, or allow a null property to be updated
        if (val == null) {
          target[key] = value;
        } else if (typeof val === 'object' && val !== null && typeof value === 'object' && value !== null) {
          defaultsDeep(val, value);
        }
      }
    }
  }

  var len = arguments.length, i = 0;
  while (i < len) {
    var obj = arguments[i++];
    if (obj) {
      copy(target, obj);
    }
  }
  return target;
};