I am working through an AWS guide for Cognito.

The Lambda function calls context.done() with two parameters, null and event.

I am trying to understand what the null parameter is and what context.done is doing with the event being passed in to it.

exports.handler = (event, context) => {
  if (event.request.session.length === 1 && 
    event.request.session[0].challengeName === 'SRP_A') {
    event.response.issueTokens = false;
    event.response.failAuthentication = false;
    event.response.challengeName = 'CUSTOM_CHALLENGE';
  } else if (
    event.request.session.length === 2 &&
    event.request.session[1].challengeName === 'CUSTOM_CHALLENGE' &&
    event.request.session[1].challengeResult === true
  ) {
    event.response.issueTokens = true;
    event.response.failAuthentication = false;
  } else {
    event.response.issueTokens = false;
    event.response.failAuthentication = true;
  }
  context.done(null, event);
};

I have console logged the context object, but I still don't understand what it's doing.

2022-05-04T22:16:59.591Z    4c2609e6-69d7-4bd9-a25c-8527b7c6653a    INFO    {
  callbackWaitsForEmptyEventLoop: [Getter/Setter],
  succeed: [Function (anonymous)],
  fail: [Function (anonymous)],
  done: [Function (anonymous)],
  functionVersion: '$LATEST',
  functionName: 'cognito-define-auth-challenge',
  memoryLimitInMB: '128',
  logGroupName: '/aws/lambda/cognito-define-auth-challenge',
  logStreamName: '2022/05/04/[$LATEST]280e88c6f58545f8bc6e5d177c13cc5b',
  clientContext: undefined,
  identity: undefined,
  invokedFunctionArn: 'arn:aws:lambda:us-east-1:268280247265:function:cognito-define-auth-challenge',
  awsRequestId: '4c2609e6-69d7-4bd9-a25c-8527b7c6653a',
  getRemainingTimeInMillis: [Function: getRemainingTimeInMillis]
}