The best way to write unit checks when utilizing the AWS JavaScript SDK v3?

Writing unit checks for code that interacts with the AWS JavaScript SDK v3 comes with two main advantages. Clearly, writing unit checks ensures you catch bugs early and due to this fact enhance the standard of your code. Additionally, writing unit checks lets you run your code domestically with out the necessity to attain out to the AWS service APIs. However how do you write unit checks for code interacting with the AWS JavaScript SDK v3?

Within the following, I’ll share my learnings from writing unit checks by utilizing aws-sdk-client-mock by Maciej Radzikowski.

aws-sdk-client-mock is straightforward to make use of!

Let’s begin with a easy instance.

The next code snippet reveals the index.js file containing a handler() operate, which may very well be deployed as a Lambda operate. The handler() operate lists all buckets belonging to an AWS account.

import { S3Client, ListBucketsCommand } from '@aws-sdk/client-s3';

export async operate handler(occasion, context) {
  const s3Client = new S3Client();
  const response = await s3Client.ship(new ListBucketsCommand({}));
  return response.Buckets;
}

So, how do I write a unit take a look at? I desire utilizing the take a look at framework mocha to put in writing JavaScript checks. The next snippet reveals the take a look at/take a look at.index.js file containing the skeleton to implement a take a look at.

import { deepStrictEqual } from 'node:assert';

import { handler } from '../index.js';

describe('demo', () => {
  it('handler', async () => {
    const now = new Date().toISOString();
    const consequence = await handler({}, {}); 
    deepStrictEqual(consequence, [{ 
      Name: 'bucket-demo-1',
      CreationDate: now
    }])
  });
});

When executing the take a look at, the handler() operate will ship a request to the S3 API. However doing so shouldn’t be possible for unit testing, as it is vitally difficult to make sure the response matches the assumptions within the unit take a look at.

As an alternative of sending requests to the AWS APIs use a typical testing method known as mocking. A mock simulates a dependency. So let’s mock the AWS Java Script SDK v3 by extending the take a look at/take a look at.index.js file.

import { S3Client, ListBucketsCommand } from '@aws-sdk/client-s3';
import { mockClient } from 'aws-sdk-client-mock';
import { deepStrictEqual } from 'node:assert';

import {handler} from '../index.js';

const s3Mock = mockClient(S3Client); 

beforeEach(() => {
  s3Mock.reset(); 
});

describe('demo', () => {
  it('handler', async () => {
    const now = new Date().toISOString();
    s3Mock.on(ListBucketsCommand).resolvesOnce({ 
      Buckets: [{
        Name: 'bucket-demo-1',
        CreationDate: now
      }]
    });
    const consequence = await handler({}, {});
    deepStrictEqual(consequence, [{
      Name: 'bucket-demo-1',
      CreationDate: now
    }]);
  });
});

Need to run the instance your self? Right here is the bundle.json that it’s essential to setup the instance.

{
  "dependencies": {
    "@aws-sdk/client-s3": "^3.583.0",
    "aws-sdk-client-mock": "^4.0.0"
  },
  "title": "aws-mock-demo",
  "model": "1.0.0",
  "major": "index.js",
  "devDependencies": {
    "aws-sdk-client-mock": "^4.0.0",
    "mocha": "^10.2.0"
    
  },
  "scripts": {
    "take a look at": "mocha"
  },
  "kind": "module",
  "writer": "",
  "license": "ISC",
  "description": ""
}

Lastly, run the take a look at.

> aws-mock-demo@1.0.0 take a look at
> mocha
  demo
    ✔ handler

  1 passing (5ms)

const s3Client = new S3Client({});
const s3Mock = mockClient(s3Client);

const s3Mock = mockClient(S3Client);

import { DynamoDBClient, paginateScan } from '@aws-sdk/client-dynamodb';

const dynamodbClient = new DynamoDBClient();

export async operate handler(occasion, context) {
  let consequence = '';
  const paginator = paginateScan({ 
    shopper: dynamodbClient
  }, {
    TableName: 'demo'
  });
  for await (const web page of paginator) {
    consequence = consequence + web page.Objects[0].Information.S;
  }
  return consequence;
}

import { DynamoDBClient, ScanCommand } from '@aws-sdk/client-dynamodb';
import { mockClient } from 'aws-sdk-client-mock';
import { deepStrictEqual } from 'node:assert';

const dynamodbMock = mockClient(DynamoDBClient);
import { handler } from '../index.js';

beforeEach(() => {
  dynamodbMock.reset();
});

describe('demo', () => {
  it('handler', async () => {
    dynamodbMock.on(ScanCommand, {ExclusiveStartKey: undefined}).resolvesOnce({
      Objects: [{Key: {S: '1'}, Data: {S: 'Hello '}}],
      Rely: 1,
      LastEvaluatedKey: {Key: {S: '1'}}
    });
    dynamodbMock.on(ScanCommand, {ExclusiveStartKey: {Key: {S: '1'}}}).resolvesOnce({
      Objects: [{Key: {S: '2'}, Data: {S: 'World'}}],
      Rely: 1,
      LastEvaluatedKey: {Key: {S: '2'}}
    });
    dynamodbMock.on(ScanCommand, {ExclusiveStartKey: {Key: {S: '2'}}}).resolvesOnce({
      Objects: [{Key: {S: '3'}, Data: {S: '!'}}],
      Rely: 1
    });
    const consequence = await handler({}, {});
    deepStrictEqual(consequence, 'Hiya World!');
  });
});