Utils file for merging streaming todos

index.js

class Utils {
  static mergeTodos(todos, streamData) {
    // generate an array of ids of streamData
    const streamDataIds = streamData.map(todo => todo._id);

    return (
      todos
        // consider streamData as the source of truth
        // first take existing todos which are not present in stream data
        .filter(({ _id }) => !streamDataIds.includes(_id))
        // then add todos from stream data
        .concat(streamData)
        // remove todos which are deleted in stream data
        .filter(todo => !todo._deleted)
        // finally sort on the basis of creation timestamp
        .sort((a, b) => a.createdAt - b.createdAt)
    );
  }
}

export default Utils;