How Node.js mocking actually works

failing.test.js

const AWSMock = require('aws-sdk-mock');
const AWS = require('aws-sdk');
const expect = require('chai').expect;

describe('the module', () => {
  it('should mock getItem from DynamoDB', async () => {
    // Here we initiate the AWS.DynamoDB object before we call AWSMock
    const dynamodb = new AWS.DynamoDB({ apiVersion: '2012-08-10' });

    AWSMock.setSDKInstance(AWS);

    // Since the object already exists in memory, this doesn't work
    AWSMock.mock('DynamoDB', 'getItem', (params, callback) => {
      console.log('DynamoDB', 'getItem', 'mock called');
      callback(null, { pk: 'foo', sk: 'bar' });
    });

    let input = { TableName: '', Key: {} };

    // In here we will call the ACTUAL AWS SDK because it was initiated before the mock, making this unit test fail
    const result = await dynamodb.getItem(input).promise();

    expect(result).to.eql({ pk: 'foo', sk: 'bar' });

    AWSMock.restore('DynamoDB');
  });
});

working.test.js

const AWSMock = require('aws-sdk-mock');
const AWS = require('aws-sdk');
const expect = require('chai').expect;

describe('the module', () => {
  it('should mock getItem from DynamoDB', async () => {
    AWSMock.setSDKInstance(AWS);
    
    // Mocking DynamoDB.getItem()
    AWSMock.mock('DynamoDB', 'getItem', (params, callback) => {
      console.log('DynamoDB', 'getItem', 'mock called');
      callback(null, { pk: 'foo', sk: 'bar' });
    });

    let input = { TableName: '', Key: {} };
    
    // When calling AWS.DynamoDB node.js will use the mocked object
    const dynamodb = new AWS.DynamoDB({ apiVersion: '2012-08-10' });
    
    const result = await dynamodb.getItem(input).promise();
    expect(result).to.eql({ pk: 'foo', sk: 'bar' });

    AWSMock.restore('DynamoDB');
  });
});