Mocking function to unit test Serverless Lambda - node.js

Question

I am really struggling to understand unit testing within a Serverless Application. So I obviously have my handler, and I have a single Lambda function
const responses = require('../utils/jsonResponse');
const someConnector = require('../services/connectToService/connectToService');
module.exports = async (event) => {
const connectionParams = {
//some env variables
};
try {
const token = await someConnector.connectToService(connectionParams);
return responses.status(token, 200);
} catch (e) {
return responses.status(
`Issue connecting to service - ${e.message}`,
500,
);
}
};
So this Lambda function is pretty straight forward, gets some environment variables, and awaits a response from a service. It then returns the response.
So I have already done integration tests for this which is fine, but now I wanted to do a Unit test. I wanted to test this function in isolation, so essentially I want to mock connectToService to return my own responses.
So I came up with the following
require('dotenv').config();
const { expect } = require('chai');
const sinon = require('sinon');
let sandbox = require("sinon").createSandbox();
const LambdaTester = require('lambda-tester');
const handler = require('../../../handler');
const msConnector = require('../../../services/connectToService/connectToService');
describe('Testing handler', async (done) => {
describe('endpoint someEndpoint returns 200', () => {
it('Should resolve with 200', async () => {
before(() => {
sandbox = sinon.createSandbox();
sandbox.stub(msConnector, 'connectToService').resolves('some-token');
});
afterEach(() => {
sandbox.restore();
});
await LambdaTester(handler.someEndpoint)
.expectResult((result) => {
console.log(result);
expect(result.statusCode).to.equal(200);
});
});
});
done();
});
msConnector is the filename of the service, connectToService is the function name. What I want to do is not invoke this function, but return some-token when my Lambda calls it.
However, I have the console.log, and what I get from that is the real token, not some-token.
This tells me that the mocked function is really being called and executed and returning the real value.
So how can I mock this to make sure it returns some-token?
Thanks
Service function
const { DOMParser } = require('#xmldom/xmldom');
const axios = require('axios');
const { loginRequest } = require('./xml/login');
const connectToService = async (connectionParams) => {
//this injects config details into XML
const xmlRequest = loginRequest(
connectionParams.username,
connectionParams.password,
connectionParams.url,
);
const config = {
headers: {
'Content-Type': 'application/x-www-form-urlencoded',
'Content-Length': xmlRequest.length,
},
};
const token = await axios
.post(connectionParams.msHost, xmlRequest, config)
.then((res) => {
const dom = new DOMParser().parseFromString(res.data, 'text/xml');
if (
dom.documentElement
.getElementsByTagName('wsse:secToken')
.item(0)
) {
return dom.documentElement
.getElementsByTagName('wsse:secToken')
.item(0).firstChild.nodeValue;
}
throw new Error('Invalid Username/Password');
})
.catch((err) => {
throw new Error(`Error making connection - ${err.message}`);
});
return token;
};
module.exports = {
connectToService,
};

Answer 1

The function connectToService may be not same copy between you mocked and called.
Because you overwrote a new object by module.exports = .... This causes you probably get different object for each require.
Try to do the below approach sharing the same object for all require.
const { DOMParser } = require('#xmldom/xmldom');
const axios = require('axios');
const { loginRequest } = require('./xml/login');
const connectToService = async (connectionParams) => {
//this injects config details into XML
const xmlRequest = loginRequest(
connectionParams.username,
connectionParams.password,
connectionParams.url,
);
const config = {
headers: {
'Content-Type': 'application/x-www-form-urlencoded',
'Content-Length': xmlRequest.length,
},
};
const token = await axios
.post(connectionParams.msHost, xmlRequest, config)
.then((res) => {
const dom = new DOMParser().parseFromString(res.data, 'text/xml');
if (
dom.documentElement
.getElementsByTagName('wsse:secToken')
.item(0)
) {
return dom.documentElement
.getElementsByTagName('wsse:secToken')
.item(0).firstChild.nodeValue;
}
throw new Error('Invalid Username/Password');
})
.catch((err) => {
throw new Error(`Error making connection - ${err.message}`);
});
return token;
};
module.exports.connectToService = connectToService;