JEST unit testing example for fs.readFile on Lambda handler

createErrorBody.js

module.exports = (data) => {
  const {
    message, path, reason, requestId, code,
  } = data;

  return {
    message,
    error: {
      path,
      reason,
      requestId,
      code,
    },
  };
};

customMessageForgotPasswordTrigger.json

{
  "version": 1,
  "triggerSource": "CustomMessage_ForgotPassword",
  "region": "us-west-2",
  "userPoolId": "us-west-2_Rdf128test23r4",
  "userName": "testUser",
  "callerContext": {
      "awsSdk": "1",
      "clientId": "123456789"
  },
  "request": {
      "userAttributes": {
          "phone_number_verified": false,
          "email_verified": true
      },
      "codeParameter": "test"
  },
  "response": {
      "smsMessage": "<custom message to be sent in the message with code parameter>",
      "emailMessage": "<custom message to be sent in the message with code parameter>",
      "emailSubject": "<custom email subject>"
  }
}

sendCustomMessage.js

const fs = require('fs');
const createErrorBody = require('./utils/createErrorBody');

async function readCustomForgotPasswordEmail() {
  return new Promise((resolve, reject) => {
    fs.readFile('./emails/forgotpassword-email.html', 'utf-8', (err, data) => {
      if (err) {
        console.log(JSON.stringify(createErrorBody({
          message: 'Not able to read the forgotpassword custom email',
          reason: 'Not able to read the forgotpassword custom email',
          code: '4006',
        })));
        reject(err);
      } else {
        console.log('Forgot Password Custom Email Read Successfully');
        resolve(data);
      }
    });
  });
}

exports.handler = async (event) => {
  const newEvent = { ...event };
  // Identify why was this function invoked
  if (event.triggerSource === 'CustomMessage_ForgotPassword') {
    // Ensure that your message contains event.request.codeParameter
    let emailMessage;
    try {
      emailMessage = await readCustomForgotPasswordEmail();
    } catch (e) {
      return event;
    }
    newEvent.response.emailSubject = 'Forgot Your Password Security Confirmation';
    newEvent.response.emailMessage = emailMessage;
  }
  // Create custom message for other events

  return newEvent;
};

sendCustomMessage.test.js

/* global jest describe beforeEach afterAll it expect */
const fs = require('fs');
const mod = require('../../../src/sendCustomMessage');
const lambdaWrapper = require('lambda-wrapper');

const wrapped = lambdaWrapper.wrap(mod, {
  handler: 'handler',
});

describe('sendCustomMessage', () => {
  let event = {};
  const forgotPasswordEmailFile = './emails/forgotpassword-email.html';
  const wrongForgotPasswordEmailFile = './emails/forgotpasswordemail.html';

  beforeEach(() => {
    event = JSON.parse(fs.readFileSync('./test/testData/customMessageForgotPasswordTrigger.json'));
  });

  afterAll(() => {
    fs.renameSync(wrongForgotPasswordEmailFile, forgotPasswordEmailFile);
  });

  it('should return with email content when having successfully read forgotpassword email template', () => {
    return wrapped.run(event).then((response) => {
      expect(response.response.emailSubject).toEqual('Forgot Your Password Security Confirmation');
    });
  });
  
  it('should mock console.log and equal "Forgot Password Custom Email Read Successfully"', () => {
    // tests the console.log in the resolve else branch
    console.log = jest.fn();
    return wrapped.run(event).then(() => {
      expect(console.log.mock.calls[0][0]).toEqual('Forgot Password Custom Email Read Successfully');
    });
  });

  it('should return without email content when forgotpassword email failed to be read', () => {
    fs.renameSync(forgotPasswordEmailFile, wrongForgotPasswordEmailFile);
    return wrapped.run(event).then((response) => {
      expect(response.response.emailSubject).toEqual('<custom email subject>');
    });
  });
});