aws-sdk-mock

AWSome mocks for Javascript aws-sdk services.

This module was created to help test AWS Lambda functions but can be used in any situation where the AWS SDK needs to be mocked.

If you are new to Amazon WebServices Lambda (or need a refresher), please checkout our our
Beginners Guide to AWS Lambda: https://github.com/dwyl/learn-aws-lambda

• Why
• What
• Getting Started
• Documentation
• Background Reading

Why?

Testing your code is essential everywhere you need reliability.

Using stubs means you can prevent a specific method from being called directly. In our case we want to prevent the actual AWS services to be called while testing functions that use the AWS SDK.

What?

Uses Sinon.js under the hood to mock the AWS SDK services and their associated methods.

How? (Usage)

install aws-sdk-mock from NPM

npm install aws-sdk-mock --save-dev

Use in your Tests

Using plain JavaScript

const AWS = require('aws-sdk-mock');

AWS.mock('DynamoDB', 'putItem', function (params, callback){
  callback(null, 'successfully put item in database');
});

AWS.mock('SNS', 'publish', 'test-message');

// S3 getObject mock - return a Buffer object with file data
AWS.mock('S3', 'getObject', Buffer.from(require('fs').readFileSync('testFile.csv')));


/**
    TESTS
**/

AWS.restore('SNS', 'publish');
AWS.restore('DynamoDB');
AWS.restore('S3');
// or AWS.restore(); this will restore all the methods and services

Using TypeScript

import AWSMock from 'aws-sdk-mock';
import AWS from 'aws-sdk';
import { GetItemInput } from 'aws-sdk/clients/dynamodb';

beforeAll(async (done) => {
  //get requires env vars
  done();
 });

describe('the module', () => {

/**
    TESTS below here
**/

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

    const input:GetItemInput = { TableName: '', Key: {} };
    const dynamodb = new AWS.DynamoDB({apiVersion: '2012-08-10'});
    expect(await dynamodb.getItem(input).promise()).toStrictEqual({ pk: 'foo', sk: 'bar' });

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

  it('should mock reading from DocumentClient', async () => {
    // Overwriting DynamoDB.DocumentClient.get()
    AWSMock.setSDKInstance(AWS);
    AWSMock.mock('DynamoDB.DocumentClient', 'get', (params: GetItemInput, callback: Function) => {
      console.log('DynamoDB.DocumentClient', 'get', 'mock called');
      callback(null, {pk: 'foo', sk: 'bar'});
    });

    const input:GetItemInput = { TableName: '', Key: {} };
    const client = new AWS.DynamoDB.DocumentClient({apiVersion: '2012-08-10'});
    expect(await client.get(input).promise()).toStrictEqual({ pk: 'foo', sk: 'bar' });

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

Sinon

You can also pass Sinon spies to the mock:

const updateTableSpy = sinon.spy();
AWS.mock('DynamoDB', 'updateTable', updateTableSpy);

// Object under test
myDynamoManager.scaleDownTable();

// Assert on your Sinon spy as normal
assert.isTrue(updateTableSpy.calledOnce, 'should update dynamo table via AWS SDK');
const expectedParams = {
  TableName: 'testTableName',
  ProvisionedThroughput: {
    ReadCapacityUnits: 1,
    WriteCapacityUnits: 1
  }
};
assert.isTrue(updateTableSpy.calledWith(expectedParams), 'should pass correct parameters');

NB: The AWS Service needs to be initialised inside the function being tested in order for the SDK method to be mocked e.g for an AWS Lambda function example 1 will cause an error ConfigError: Missing region in config whereas in example 2 the sdk will be successfully mocked.

Example 1:

const AWS      = require('aws-sdk');
const sns      = AWS.SNS();
const dynamoDb = AWS.DynamoDB();

exports.handler = function(event, context) {
  // do something with the services e.g. sns.publish
}

Example 2:

const AWS = require('aws-sdk');

exports.handler = function(event, context) {
  const sns      = AWS.SNS();
  const dynamoDb = AWS.DynamoDB();
  // do something with the services e.g. sns.publish
}

Also note that if you initialise an AWS service inside a callback from an async function inside the handler function, that won't work either.

Example 1 (won't work):

exports.handler = function(event, context) {
  someAsyncFunction(() => {
    const sns      = AWS.SNS();
    const dynamoDb = AWS.DynamoDB();
    // do something with the services e.g. sns.publish
  });
}

Example 2 (will work):

exports.handler = function(event, context) {
  const sns      = AWS.SNS();
  const dynamoDb = AWS.DynamoDB();
  someAsyncFunction(() => {
    // do something with the services e.g. sns.publish
  });
}

Nested services

It is possible to mock nested services like DynamoDB.DocumentClient. Simply use this dot-notation name as the service parameter to the mock() and restore() methods:

AWS.mock('DynamoDB.DocumentClient', 'get', function(params, callback) {
  callback(null, {Item: {Key: 'Value'}});
});

NB: Use caution when mocking both a nested service and its parent service. The nested service should be mocked before and restored after its parent:

// OK
AWS.mock('DynamoDB.DocumentClient', 'get', 'message');
AWS.mock('DynamoDB', 'describeTable', 'message');
AWS.restore('DynamoDB');
AWS.restore('DynamoDB.DocumentClient');

// Not OK
AWS.mock('DynamoDB', 'describeTable', 'message');
AWS.mock('DynamoDB.DocumentClient', 'get', 'message');

// Not OK
AWS.restore('DynamoDB.DocumentClient');
AWS.restore('DynamoDB');

Don't worry about the constructor configuration

Some constructors of the aws-sdk will require you to pass through a configuration object.

const csd = new AWS.CloudSearchDomain({
  endpoint: 'your.end.point',
  region: 'eu-west'
});

Most mocking solutions with throw an InvalidEndpoint: AWS.CloudSearchDomain requires an explicit 'endpoint' configuration option when you try to mock this.

aws-sdk-mock will take care of this during mock creation so you won't get any configuration errors!
If configurations errors still occur it means you passed wrong configuration in your implementation.

Setting the aws-sdk module explicitly

Project structures that don't include the aws-sdk at the top level node_modules project folder will not be properly mocked. An example of this would be installing the aws-sdk in a nested project directory. You can get around this by explicitly setting the path to a nested aws-sdk module using setSDK().

Example:

const path = require('path');
const AWS = require('aws-sdk-mock');

AWS.setSDK(path.resolve('../../functions/foo/node_modules/aws-sdk'));

/**
    TESTS
**/

Setting the aws-sdk object explicitly

Due to transpiling, code written in TypeScript or ES6 may not correctly mock because the aws-sdk object created within aws-sdk-mock will not be equal to the object created within the code to test. In addition, it is sometimes convenient to have multiple SDK instances in a test. For either scenario, it is possible to pass in the SDK object directly using setSDKInstance().

Example:

// test code
const AWSMock = require('aws-sdk-mock');
import AWS from 'aws-sdk';
AWSMock.setSDKInstance(AWS);
AWSMock.mock('SQS', /* ... */);

// implementation code
const sqs = new AWS.SQS();

Configuring promises

If your environment lacks a global Promise constructor (e.g. nodejs 0.10), you can explicitly set the promises on aws-sdk-mock. Set the value of AWS.Promise to the constructor for your chosen promise library.

Example (if Q is your promise library of choice):

const AWS = require('aws-sdk-mock'),
    Q = require('q');

AWS.Promise = Q.Promise;


/**
    TESTS
**/

Documentation

AWS.mock(service, method, replace)

Replaces a method on an AWS service with a replacement function or string.

AWS.restore(service, method)

Removes the mock to restore the specified AWS service

If AWS.restore is called without arguments (AWS.restore()) then all the services and their associated methods are restored i.e. equivalent to a 'restore all' function.

AWS.remock(service, method, replace)

Updates the replace method on an existing mocked service.

AWS.setSDK(path)

Explicitly set the require path for the aws-sdk

AWS.setSDKInstance(sdk)

Explicitly set the aws-sdk instance to use

Background Reading

• Mocking using Sinon.js
• AWS Lambda

Contributions welcome! Please submit issues or PRs if you think of anything that needs updating/improving