How to unit-test a AWS Lambda use lambda-local with mocha?

Question

Background

I am working on a project that is build with Serverless framework with serverless-appsync-plugin . I implemented a single endpoint(AWS Lambda) to handle all the requests generate from Appsync through graphQL. The endpoint will route the request to the corresponding function to perform the operation.

Problem

Now that I have developed around 10+ operations, I want to automate the process of unit-testing. For simplicity, I decided to run that single endpoint as a lambda locally for all the testing (against running appsync-offline).

So, I used lambda-local with mocha . However, I am not able to get a test-case to fail, based on the response I got from the lambda.

it('db should have a user of uId: 123',  async function () {
  lambdaLocal.execute({
    event: makeUserEvent({userId: '123'}),
    callbackWaitsForEmptyEventLoop: false,
    lambdaPath,
    callback: function(err, data) {
      if(err) {
        console.log(1)
        expect.fail(null, null, 'You are not supposed to be here') //should fail here
      } else {
        console.log(2)
        // some checking here, may fail or may not fail
        expect.fail(null, null, 'return fail if the userId is 234') //should fail here too
      }
    }
  });
  console.log(3)
})

In both of the situation I want it to fail, it is not failing the test cases for either callback('failed', null) or callback(null, 'success') .

So, what is the right way to make the lambda-local to fail a test case?

Answer 1

In your code the test finishes without mocha registering that the assertions in lambdaLocal.execute failed. So it will always pass.

Instead of using the callback parameter you could return a promise, or await the lambdalocal.execute and then do your assertions.

For example (using Promises):

it('db should have a user of uId: 123',  function () {
  return lambdaLocal.execute({
    event: makeUserEvent({userId: '123'}),
    callbackWaitsForEmptyEventLoop: false,
    lambdaPath })
 .then(
    (data) => {
        console.log(2)
        // some checking here, may fail or may not fail
        expect.fail(null, null, 'return fail if the userId is 234') //should fail here
    },
    (err) => {
        console.log(1)
        expect.fail(null, null, 'You are not supposed to be here') //should fail here
    })
}

Alternatively change signature of the function that is passed to it to take an additional parameter (usually called done ) which will then be used by mocha to pass a function that can be used to signalise that the test finished. See the mocha documentation for more info.