How do I mock the QueryIterator returned by the DynamoDb Datamapper with Jest?

Question

I'm trying to write unit tests for a TypeScript class that calls DynamoDb. This is what I've got so far.

import {DataMapper, QueryIterator} from '@aws/dynamodb-data-mapper';
import DynamoDB = require("aws-sdk/clients/dynamodb");

jest.mock("aws-sdk/clients/dynamodb");
jest.mock("@aws/dynamodb-data-mapper");


describe("queryDynamoDb", () => {
    let myRepository = new MyRepository();
    const mockQueryCall = jest.fn();

    DataMapper.prototype.query = mockQueryCall;

   test("queriesDynamoDb", async () => {
        const mockQueryIterator = <jest.Mock<QueryIterator<DataObject>>>QueryIterator;
        mockQueryIterator.prototype.next.mockReturnValueOnce(getExpectedReturnObjectsValues())
        mockQueryCall.mockReturnValue(mockQueryIterator);

        let responseObjects = await myRepository.queryDynamoDb(testIdString);

        // Validate response Objects
   })
});

The method iterates through the response with a for...of loop, so I need the .next function to return.

To test the test, I added console.log(await queryIterator.next()); to my dynamoDb querying function.

But when I try to run it, the response is:

TypeError: queryIterator.next is not a function

on that log line.

So, clearly mockQueryCall.mockReturnValue(mockQueryIterator); is either not doing its work and returning my mockQueryIterator, or const mockQueryIterator = <jest.Mock<QueryIterator<DataObject>>>QueryIterator; is not correctly applying type to the mockQueryIterator. But I don't know how to fix either of those things.

Answer 1

After some trial and error, I came up with something that works. It mocks the QueryIterator's Symbol.iterator function.

import {DataMapper, QueryIterator, StringToAnyObjectMap} from '@aws/dynamodb-data-mapper';
import createMockInstance from 'jest-create-mock-instance';

describe('alert service test', () => {

    test('find by user id', async () => {
        const mapper = createMockInstance(DataMapper);
        const qi: QueryIterator<StringToAnyObjectMap> = createMockInstance(QueryIterator);
        // @ts-ignore
        qi[Symbol.iterator] = jest.fn(() => [1, 2, 3].values());
        mapper.query.mockReturnValue(qi);

        // @ts-ignore any query arguments will do
        const results = mapper.query();

        const resultArray = [];
        for await (const o of results) {
            resultArray.push(o);
        }
        expect(resultArray).toEqual([1, 2, 3]);
    });
});