react-esm/dist-src/ReactLazy.js

React is a JavaScript library for building user interfaces.

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
import { REACT_LAZY_TYPE } from 'shared/ReactSymbols';
import warning from 'shared/warning';
export function lazy(ctor) {
  let lazyType = {
    $$typeof: REACT_LAZY_TYPE,
    _ctor: ctor,
    // React uses these fields to store the result.
    _status: -1,
    _result: null
  };

  if (__DEV__) {
    // In production, this would just set it on the object.
    let defaultProps;
    let propTypes;
    Object.defineProperties(lazyType, {
      defaultProps: {
        configurable: true,

        get() {
          return defaultProps;
        },

        set(newDefaultProps) {
          warning(false, 'React.lazy(...): It is not supported to assign `defaultProps` to ' + 'a lazy component import. Either specify them where the component ' + 'is defined, or create a wrapping component around it.');
          defaultProps = newDefaultProps; // Match production behavior more closely:

          Object.defineProperty(lazyType, 'defaultProps', {
            enumerable: true
          });
        }

      },
      propTypes: {
        configurable: true,

        get() {
          return propTypes;
        },

        set(newPropTypes) {
          warning(false, 'React.lazy(...): It is not supported to assign `propTypes` to ' + 'a lazy component import. Either specify them where the component ' + 'is defined, or create a wrapping component around it.');
          propTypes = newPropTypes; // Match production behavior more closely:

          Object.defineProperty(lazyType, 'propTypes', {
            enumerable: true
          });
        }

      }
    });
  }

  return lazyType;
}