Test-Driven Serverless Application Development

Written by Eetu TuomalaEdit this post

#Quick Start to Test-Driven Development with Serverless Framework

If you don’t possess the magic that keeps your code working, the following instructions will help you start test-driven Serverless application development.

What I like most about the Serverless Framework as a development tool is that it gathers together all of the cloud resources for a structured project. With the whole stack in the same project, it's convenient and easy to start writing tests.

Usually for a new project I use the SC5 Serverless boilerplate. It's a good setup to begin with. But in this tutorial, I start with an existing example aws-node-simple-http-endpoint project, to show how easy it is to add Serverless testing plugin even to an existing project.

Let’s start by installing the service, changing the directory to the one that sls install command creates, and installing dependencies that service requires to run.

sls install -u https://github.com/serverless/examples/tree/master/aws-node-simple-http-endpoint -n my-tdd-service
cd my-tdd-service
npm install

Then install the Mocha plugin with npm install --save-dev serverless-mocha-plugin. If you're more familiar with writing tests with Jest, you can use Jest plugin.

The next step is to add the installed plugin to serverless.yml. This project doesn't have any plugins yet installed so the plugins key is also added.

  - serverless-mocha-plugin

Now, run sls and the output should include following new commands that the Mocha plugin adds to the Serverless Framework.

create test ................... Create mocha tests for service / function
create function ............... Create a function into the service
invoke test ................... Invoke test(s)

To create a test to an existing function, use the create test command with parameter -f or --function. In this example project, there is already a function called currentTime. To create a test stub for that, run sls create test -f currentTime and it should print out Serverless: serverless-mocha-plugin: created test/currentTime.js as a result.

Next, invoke the test by running sls invoke test and the output should be something like this:

    ✓ implement tests here

  1 passing (8ms)

Now for the fun part – implementing the actual tests. The tests that the Serverless Mocha plugin creates are in the test directory, which is the default directory for Mocha tests. If you prefer to use a different directory you can create tests using -p or --path parameter when creating and invoking tests. Open the tests/currentTime.js to your code editor.

There is a generated test that only tests that the response is not empty.

it('implement tests here', () => {
  return wrapped.run({}).then((response) => {

Replace that with the following one, which tests that statusCode is 200 and that the response body contains a message that has the time. In real life, you may want to fake the date with Sinon.JS or similar so that you can test the response with predefined dates.

it('should return current time', () => {
  return wrapped.run({}).then((response) => {
    const body = JSON.parse(response.body);

After invoking the tests again with sls invoke test command, the output should be:

    ✓ should return current time

  1 passing (8ms)

The first test is now ready. Let's create some more! Now we have a function that returns time, so we need to know the date also. With sls create function command you are able to create a function and test case for it.

sls create function -f currentDate --handler date/handler.endpoint

Then open test/currentDate.js to your editor and replace the default implement tests here test block with snippet:

it('should return current date', () => {
  return wrapped.run({}).then((response) => {
    const body = JSON.parse(response.body);

When invoking tests with sls invoke test command, you should get the following error:

AssertionError: expected 'Go Serverless v1.0! Your function executed successfully!'
to match 

Next fix the function to match the test, open date/handler.js and replace the code with one that returns the date.

'use strict';

module.exports.endpoint = (event, context, callback) => {
  const response = {
    statusCode: 200,
    body: JSON.stringify({
      message: `Hello, the current date is ${new Date().toDateString()}.`,

  callback(null, response);

Once again, run the sls invoke test command and the result should be successful.

In addition to test keeping your code functional, the benefit of using test instead of e.g. sls invoke local is that you can test the same handler function easily with different payloads.

If you'd like a working example, here's the repository I used while making this tutorial.

About Eetu Tuomala

Eetu Tuomala is a cloud application architect at SC5 Online Ltd., who specialises in designing and building cloud-native services.

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