Testing AWS services with Sinon mocks in Lambda

mocking-with-sinon-sandbox.js

/**
 * Writing tests for functions that use external dependencies is tough.
 * We can get past this by using sinon sandboxes to temporarily overwrite the
 * prototypes of those dependencies, just for testing
 * 
 * Before running these tests run:
 * $ npm install --save-dev aws-sdk sinon mocha
 */

/**
 * index.js - The file to be tested
 */
const { DynamoDB } = require('aws-sdk')
const dynamoDB = new DynamoDB.DocumentClient()

exports.handler = async (event) => {
	let result
	const params = {
		TableName: 'Tasks',
		Key: { id: event.pathParameters.id }
	}

	try {
		result = (await dynamoDB.get(params).promise()).Item
	} catch (e) {
		result = { error: 'task not found' }
	}

	return result
}

/**
 * index.spec.js - The test file that tests index.js
 */
const assert = require('assert') // Built in node library, no need to install
const sinon = require('sinon')
const { DynamoDB } = require('aws-sdk')
const { handler } = require('./index.js')

// create a sinon sandbox
const sandbox = sinon.createSandbox()

describe('Test my lambda', () => {
	describe('#handler()', () => {

		beforeEach(() => sandbox
			// Here I overwrite every call to #get()
			.stub(DynamoDB.DocumentClient.prototype, 'get')
			// And define my own return
			.returns({
				// promise function is required here because we use the
				// #AWS.request.promise() syntax in the handler (line 24)
				promise: () => ({ Item: { id: 42 } })
			})
		)

		// Runs after every test.  Undo our temporary
		// overwrite so the next test starts clean
		afterEach(() => sandbox.restore())

		it('gets a task from the database', async () => {
			const response = await handler({ pathParameters: { id: 42 } })
			assert.strictEqual(response.id, 42)
		})
	})
})

// and finally, run your mocha tests.  I'm using babel, so I set things up with:
// $ npm install --save-dev @babel/core @babel/preset-env @babel/register
//
// and then I run all tests with mocha by calling:
// $ ./node_modules/.bin/mocha index.spec.js -- require @babel/register
//