How to write a test for this resolver?

Question

JavaScript

Hey!
I write unit tests for GraphQl resolvers , connect to the database using Sequelize , and write tests using Jest .
How to write a test for a resolver with a promise?
resolver

…
  updateUser(root, { id, input }) {
    return models.UserModel.findById(id)              // Ищем User в БД по ID
      .then(user => user.update(input));                   // Добавляем объект с новыми значениями
  }
…

My test:

…
  it('update User', () => {
    return Mutation.updateUser(null, {
      currentId,                                          // currentId  = текущий ID юзера 
      input: {
        login: 'T-Jane',
        homeFloor: 2,
        avatarUrl: 'http://pic.com/2',
      },
    })
      .then((result) => {
        return console.log(result);
      });
  });
…

The error I am getting in the console is:

TypeError: Cannot read property 'update' of null

       7 |   updateUser(root, { id, input }) {
       8 |     return models.UserModel.findById(id)
    >  9 |       .then(user => user.update(input));
      10 |   },
      11 |   removeUser(root, { id }) {
      12 |     return models.UserModel.findById(id)

      at models.UserModel.findById.then.user (graphql/resolvers/mutation.js:9:23)
      at tryCatcher (node_modules/bluebird/js/release/util.js:16:23)
      at Promise._settlePromiseFromHandler (node_modules/bluebird/js/release/promise.js:512:31)
      at Promise._settlePromise (node_modules/bluebird/js/release/promise.js:569:18)
      at Promise._settlePromiseCtx (node_modules/bluebird/js/release/promise.js:606:10)
      at Async.Object.<anonymous>.Async._drainQueue (node_modules/bluebird/js/release/async.js:138:12)
      at Async.Object.<anonymous>.Async._drainQueues (node_modules/bluebird/js/release/async.js:143:10)
      at Immediate.Async.drainQueues [as _onImmediate] (node_modules/bluebird/js/release/async.js:17:14)

As I understand it, the test sees the string with return models.UserModel.findById(id)and returns the update null result.
There is documentation on this topic, but I didn’t quite understand what was there and how, because promises are a new topic for me. Here is a link to the Jest documentation on the topic of promises.
Thank you all for your answers!)
Give advice!