3-line JavaScript Lambda Testing

tl;dr: Import the handler, execute, check the result.

Recently my backend development has been focused on serverless technologies, specifically using AWS Lambda.

I wanted to speed up development by testing locally, so I began exploring the options for creating a local lambda execution environment.

Frameworks such as Localstack and Serverless are an option, but were a little too heavy and opinionated for what I was after. lambda-local showed promise, but I found it sullied the return object with additional properties which made assertions difficult.

I began building a solution using child_process.fork() but soon realised that I was completely overthinking the simple truth:

Lambdas are just JavaScript functions.

So, why not just treat them the same as any other function?

Let’s presume we have a lambda which capitalises a provided name and returns it:

const changeCase = require('change-case');

exports.handler = async function(event) {
  const result = { success: false, name: '' };
  if(event.name) {
    result.name = changeCase.titleCase(event.name);
    result.success = true;
  }
  return result;
}

We can add a companion capitalise.spec.js file which imports the function and executes it directly:

describe('capitalise lambda', () => {
  it('should capitalise the first letter of every word', () => {
    const handler = require('capitalise.js').handler;
    const result = handler({name: 'dan salias'});
    expect(result).toStrictEqual({name: 'Dan Salias'});
  });
});

Yes, the test uses the local execution environment. If this doesn’t match the intended lambda execution environment you can use nvm to either change the node environment for the project, or toggle the node environment for testing:

{
  //...
  "scripts": {
    "test": "nvm exec 10.16.3 npx jest",
    //...
  },
  //...
  "devDependencies": {
    //...
    "jest": "^24.9.0",
  }
}

This essentially mimics the production lambda execution process, and provides an easy way to use the testing tools you are already familiar with (in my case Jest) to locally test your lambdas before deploying.

Don’t forget that your lambdas need to be packaged with all of their dependencies (node_modules) before deploying. So you’ll what to have each lambda in it’s own directory with it’s own package.json (and/or use lambda layers for dependency management).