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aws-sdk-client-mock

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Readme

AWS SDK v3 Client mock

Easy and powerful mocking of AWS SDK v3 Clients.

Features:

🌊 fluent interface - declaring behavior is short and readable
🔍 matching options - defining mock behavior by Command type and/or its input payload
🕵️ spying - checking if Commands were actually send
🖋️ fully typed - same type control for declaring mock's behavior as when writing regular code
✅ fully tested - reliable mocks help instead of impeding

In action:

aws-client-mock-example

About AWS SDK v3
Usage
API Reference
AWS Lambda example
Caveats
- Order of mock behaviors
- Order of type and instance mocks

About AWS SDK v3

The AWS SDK for JavaScript version 3, is the new version of SDK to use in Node.js and browser. It comes with modular architecture and improved typing, thanks to being written in TypeScript.

The recommended way of using it is to create a Client and use it to send Commands.

For example, using SNS Client to publish a message to a topic looks like that:

import {PublishCommand, SNSClient} from '@aws-sdk/client-sns';

const sns = new SNSClient({});
const result = await sns.send(new PublishCommand({
  TopicArn: 'arn:aws:sns:us-east-1:111111111111:MyTopic',
  Message: 'My message',
}));

console.log(`Message published, id: ${result.MessageId}`);

This library provides an easy way to mock sending Commands and define returned results depending on the Command type and payload.

Usage

Install

npm install -D aws-sdk-client-mock

yarn add -D aws-sdk-client-mock

Import

CommonJS:

const {mockClient} = require('aws-sdk-client-mock');

TypeScript / ES6:

import {mockClient} from 'aws-sdk-client-mock';

Mock

Create mock for all instances or for given instance of the AWS SDK Client:

const snsMock = mockClient(SNSClient);

const dynamoDB = new DynamoDBClient({});
const dynamoDBMock = mockClient(dynamoDB);

By default, mocked Client#send() method returns undefined.

Using the obtained mock instance, you can specify the mock behavior on receiving various commands to send.

Specify default mock behavior:

snsMock.onAnyCommand().resolves({});

// same as:

snsMock.resolves({});

Specify mock behavior on receiving given command only:

snsMock
    .on(PublishCommand)
    .resolves({
        MessageId: '12345678-1111-2222-3333-111122223333',
    });

Specify mock behavior on receiving given command with given payload only:

snsMock
    .on(PublishCommand, {
        TopicArn: 'arn:aws:sns:us-east-1:111111111111:MyTopic',
        Message: 'My message',
    })
    .resolves({
        MessageId: '12345678-4444-5555-6666-111122223333',
    });

Not all payload parameters must be defined to match (you can force strict matching by passing third param strict: true):

snsMock
    .on(PublishCommand, {
        Message: 'My message',
    })
    .resolves({
        MessageId: '12345678-4444-5555-6666-111122223333',
    });

Specify mock behavior on receiving given payload only:

snsMock
    .onAnyCommand({
        TopicArn: 'arn:aws:sns:us-east-1:111111111111:MyTopic',
        Message: 'My message',
    })
    .resolves({
        MessageId: '12345678-4444-5555-6666-111122223333',
    });

Multiple behaviors (for different commands and payloads) may be specified for a single mock:

snsMock
    .resolves({ // default for any command
        MessageId: '12345678-1111-2222-3333-111122223333'
    })
    .on(PublishCommand)
    .resolves({ // default for PublishCommand
        MessageId: '12345678-4444-5555-6666-111122223333'
    })
    .on(PublishCommand, {
        TopicArn: 'arn:aws:sns:us-east-1:111111111111:MyTopic',
        Message: 'My message',
    })
    .resolves({ // for PublishCommand with given input
        MessageId: '12345678-7777-8888-9999-111122223333',
    });

Specify mock throwing an error:

snsMock
    .rejects('mocked rejection');

Specify custom mock function:

snsMock
    .callsFake(input => {
        if (input.Message === 'My message') {
            return {MessageId: '12345678-1111-2222-3333-111122223333'};
        } else {
            return {MessageId: '12345678-4444-5555-6666-111122223333'};
        }
    });

Inspect received calls:

snsMock.calls(); // all received calls
snsMock.call(0); // first received call

Under the hood, the library uses Sinon.js stub. You can get the stub instance to configure and use it directly:

const snsSendStub = snsMock.send;

DynamoDB DocumentClient

You can mock the DynamoDBDocumentClient just like any other Client:

import {DynamoDBDocumentClient, QueryCommand} from '@aws-sdk/lib-dynamodb';

const ddbMock = mockClient(DynamoDBDocumentClient);
ddbMock.on(QueryCommand).resolves({
    Items: [{pk: 'a', sk: 'b'}],
});

Paginated operations

To mock a paginated operation results, simply mock the corresponding Command:

import {DynamoDBClient, paginateQuery, QueryCommand} from '@aws-sdk/client-dynamodb';
import {marshall} from '@aws-sdk/util-dynamodb';

const dynamodbMock = mockClient(DynamoDBClient);
dynamodbMock.on(QueryCommand).resolves({
    Items: [
        marshall({pk: 'a', sk: 'b'}),
        marshall({pk: 'c', sk: 'd'}),
    ],
});

const dynamodb = new DynamoDBClient({});
const paginator = paginateQuery({client: dynamodb}, {TableName: 'mock'});

const items = [];
for await (const page of paginator) {
    items.push(...page.Items || []);
}

API Reference

See the full API Reference.

AWS Lambda example

Example below uses Jest as a test framework, but mocks will work with any testing library.

Let's take a simple Lambda function that takes a list of messages, sends them to SNS topic and returns message IDs:

import {PublishCommand, SNSClient} from '@aws-sdk/client-sns';

const snsTopicArn = process.env.SNS_TOPIC_ARN || '';

const sns = new SNSClient({});

export const handler = async (event: Event): Promise<string[]> => {
  const promises = event.messages.map(async (msg, idx) => {
    const publish = await sns.send(new PublishCommand({
      TopicArn: snsTopicArn,
      Message: msg,
    }));
    return publish.MessageId!;
  });

  return await Promise.all(promises);
};

interface Event {
  messages: string[];
}

Then the tests could look like this:

import {mockClient} from 'aws-sdk-client-mock';
import {PublishCommand, SNSClient} from '@aws-sdk/client-sns';
import {handler} from '../src';

const snsMock = mockClient(SNSClient);

/**
 * To be sure that unit tests are independent from each other,
 * reset mock behavior between the tests.
 */
beforeEach(() => {
  snsMock.reset();
});

it('message IDs are returned', async () => {
  snsMock.on(PublishCommand).resolves({
    MessageId: '12345678-1111-2222-3333-111122223333',
  });

  const result = await handler({
    messages: ['one', 'two', 'three']
  });

  expect(result).toHaveLength(3);
  expect(result[0]).toBe('12345678-1111-2222-3333-111122223333');
});

it('SNS Client is called', async () => {
  snsMock.on(PublishCommand).resolves({
    MessageId: '111-222-333',
  });

  await handler({
    messages: ['qq', 'xx']
  });

  expect(snsMock.calls()).toHaveLength(2);
});

For more examples, see the unit tests.

Compatibility

See compatibility table for AWS SDK v3 Clients.

Caveats

Order of mock behaviors

Wider Command matchers must be declared first, otherwise, they will take precedence over previous ones.

In this case, all PublishCommand sends will return message ID 222:

snsMock
  .on(PublishCommand, myInput).resolves({MessageId: '111'})
  .on(PublishCommand).resolves({MessageId: '222'});

If the order of the declarations is switched, sends with input matching myInput will return ID 111 and all others 222.

It works similarly with onAnyCommand().

Order of type and instance mocks

When you create both a Client type mock and a specific Client instance mock(s), you need to declare type mock last. Otherwise, the other instances will not be mocked.

Right now if you create a mock for the Client type, and then mock a specific instance of this Client, with the order of mocking as here:

const sns1 = new SNSClient({}); // not mocked

mockClient(SNSClient).resolves({MessageId: '123'});

const sns2 = new SNSClient({}); // mocked
mockClient(sns2).resolves({MessageId: '456'});

const sns3 = new SNSClient({}); // not mocked

Declaring mocks in this order will fix it:

const sns1 = new SNSClient({}); // mocked - default

const sns2 = new SNSClient({}); // mocked
mockClient(sns2).resolves({MessageId: '456'});

mockClient(SNSClient).resolves({MessageId: '123'});

const sns3 = new SNSClient({}); // mocked - default

PRs to fix this are welcome.

aws-sdk-client-mock