Applications wishing to test a serverless function locally have one of the following options:

1.       Invoke Serverless Application Model (SAM) from the AWS public cloud command line interfaces and the steps are:

a.       sam build -t template.yml

b.       sam local invoke

The command to build the libraries for a default index.js lambda function handler comes with the option to create a container image. This can be very useful for portability and the image can be executed on any container framework.

While this is optional, the local invocation might require that all the source code and its dependencies be transpiled into a format that is easy to interpret and run.

Enter Babel, a well-known transpiler for this purpose and it is almost easy to invoke different formats of javascripts and typescript notations.

2.       Another option to test the lambda function handlers has been to write different unit tests and integration tests. These tests can be run with Jest – a test framework that makes it easy to execute the tests by discovering it from the build folder. All test files are usually named as .test.js or .spec.js and the test cases follow the describe/it readable specifications that have made unit-tests a pleasure to read and execute.

Jest is local to the node_modules folder where all the dependencies of the lambda function handler are installed. It can be installed with –save-dev option that eliminates the need to install it for production builds.

Executing the jest requires a mock library for unit tests and by virtue of the Javascript’s support for object’s prototype, features can be inherited and substituted.

3.       A final option is to use a middleware that exercises just the handler methods of the lambda function handler, so that they can be invoked across the wire by curl commands. Enter Koa – a lightweight middleware that is not bundled with anything else, and writing an http server becomes as simple as:

const Koa = require('koa');

const app = new Koa();

 

// response

app.use(ctx => {

  ctx.body = 'Hello Koa';

});

 

app.listen(3000);

 

and curl http://localhost:3000/ will return ‘Hello Koa’

This comes with the nice benefit that each of the handler method can now be tried individually, and the results will be similar to how the lambda functions are invoked.

 

Another way to exercise the methods would be to include the routes:

var koa = require('koa');
var http = require('http');
var router = require('koa-router')();
var bodyParser = require('koa-body')();

router.post('/resource', bodyParser, function *(next){
  console.log(this.request.body);
  this.status = 200;
  this.body = 'some output for post requests';
  yield(next);
});

startServerOne();

function startServerOne() {
  var app = koa();
  app.use(router.routes());
  http.createServer(app.callback()).listen(8081);
  console.log('Server at Port 8081');
}

These are some of the ways to test lambda function handlers.