export function polyfill()

in index.js [50:156]
63 lines of code
19 McCabe index (conditional complexity)

export function polyfill(Component) {
  var prototype = Component.prototype;

  if (!prototype || !prototype.isReactComponent) {
    throw new Error('Can only polyfill class components');
  }

  if (
    typeof Component.getDerivedStateFromProps !== 'function' &&
    typeof prototype.getSnapshotBeforeUpdate !== 'function'
  ) {
    return Component;
  }

  // If new component APIs are defined, "unsafe" lifecycles won't be called.
  // Error if any of these lifecycles are present,
  // Because they would work differently between older and newer (16.3+) versions of React.
  var foundWillMountName = null;
  var foundWillReceivePropsName = null;
  var foundWillUpdateName = null;
  if (typeof prototype.componentWillMount === 'function') {
    foundWillMountName = 'componentWillMount';
  } else if (typeof prototype.UNSAFE_componentWillMount === 'function') {
    foundWillMountName = 'UNSAFE_componentWillMount';
  }
  if (typeof prototype.componentWillReceiveProps === 'function') {
    foundWillReceivePropsName = 'componentWillReceiveProps';
  } else if (typeof prototype.UNSAFE_componentWillReceiveProps === 'function') {
    foundWillReceivePropsName = 'UNSAFE_componentWillReceiveProps';
  }
  if (typeof prototype.componentWillUpdate === 'function') {
    foundWillUpdateName = 'componentWillUpdate';
  } else if (typeof prototype.UNSAFE_componentWillUpdate === 'function') {
    foundWillUpdateName = 'UNSAFE_componentWillUpdate';
  }
  if (
    foundWillMountName !== null ||
    foundWillReceivePropsName !== null ||
    foundWillUpdateName !== null
  ) {
    var componentName = Component.displayName || Component.name;
    var newApiName =
      typeof Component.getDerivedStateFromProps === 'function'
        ? 'getDerivedStateFromProps()'
        : 'getSnapshotBeforeUpdate()';

    throw Error(
      'Unsafe legacy lifecycles will not be called for components using new component APIs.\n\n' +
        componentName +
        ' uses ' +
        newApiName +
        ' but also contains the following legacy lifecycles:' +
        (foundWillMountName !== null ? '\n  ' + foundWillMountName : '') +
        (foundWillReceivePropsName !== null
          ? '\n  ' + foundWillReceivePropsName
          : '') +
        (foundWillUpdateName !== null ? '\n  ' + foundWillUpdateName : '') +
        '\n\nThe above lifecycles should be removed. Learn more about this warning here:\n' +
        'https://fb.me/react-async-component-lifecycle-hooks'
    );
  }

  // React <= 16.2 does not support static getDerivedStateFromProps.
  // As a workaround, use cWM and cWRP to invoke the new static lifecycle.
  // Newer versions of React will ignore these lifecycles if gDSFP exists.
  if (typeof Component.getDerivedStateFromProps === 'function') {
    prototype.componentWillMount = componentWillMount;
    prototype.componentWillReceiveProps = componentWillReceiveProps;
  }

  // React <= 16.2 does not support getSnapshotBeforeUpdate.
  // As a workaround, use cWU to invoke the new lifecycle.
  // Newer versions of React will ignore that lifecycle if gSBU exists.
  if (typeof prototype.getSnapshotBeforeUpdate === 'function') {
    if (typeof prototype.componentDidUpdate !== 'function') {
      throw new Error(
        'Cannot polyfill getSnapshotBeforeUpdate() for components that do not define componentDidUpdate() on the prototype'
      );
    }

    prototype.componentWillUpdate = componentWillUpdate;

    var componentDidUpdate = prototype.componentDidUpdate;

    prototype.componentDidUpdate = function componentDidUpdatePolyfill(
      prevProps,
      prevState,
      maybeSnapshot
    ) {
      // 16.3+ will not execute our will-update method;
      // It will pass a snapshot value to did-update though.
      // Older versions will require our polyfilled will-update value.
      // We need to handle both cases, but can't just check for the presence of "maybeSnapshot",
      // Because for <= 15.x versions this might be a "prevContext" object.
      // We also can't just check "__reactInternalSnapshot",
      // Because get-snapshot might return a falsy value.
      // So check for the explicit __reactInternalSnapshotFlag flag to determine behavior.
      var snapshot = this.__reactInternalSnapshotFlag
        ? this.__reactInternalSnapshot
        : maybeSnapshot;

      componentDidUpdate.call(this, prevProps, prevState, snapshot);
    };
  }

  return Component;
}