Mocking serverless-mysql using sinon - node.js

Question

I am trying to test an AWS lambda function (node.js) created using AWS SAM. My function uses the npm module serverless-mysql to connect to Aurora. The following are the relevant parts of my lambda function:
const connection = require('serverless-mysql')({
config: {
host : process.env.DB_HOST,
user : process.env.DB_USER,
password : process.env.DB_PASSWORD
}
});
exports.lambdaHandler = async (event, context) => {
try {
const name = event.pathParameters.name;
const rows = await connection.query('SELECT * FROM users WHERE name = ?', [name]);
await connection.end()
const user = rows[0];
return {
'statusCode': 200,
'body': JSON.stringify({
firstName: user.first_name,
lastName: user.last_name,
bk: user.bk,
team: user.current_team
})
}
} catch (err) {
console.log(err);
return err;
}
};
I am trying to tests this by mocking the serverless-mysql dependency, but I am currently unable to do so. My test looks like this:
const app = require('../../app.js');
const chai = require('chai');
const sinon = require('sinon');
const expect = chai.expect;
const event = {
pathParameters: {
name: 'johndoe'
}
}
var context;
var successConnectionObject = {
connect: function() {
return Promise.resolve();
},
query: function(sqlQuery, params) {
return Promise.resolve('');
},
end: function() {}
}
var mysql = require('serverless-mysql');
var stub = sinon.stub(mysql, 'connect').returns(successConnectionObject);
describe('Tests', function () {
it('verifies successful response', async () => {
const result = await app.lambdaHandler(event, context);
expect(result).to.be.an('object');
expect(result.statusCode).to.equal(200);
mock.verify();
mock.restore();
});
});
However, this returns the following error:
TypeError: Cannot stub non-existent own property connect
I believe this is because mysql is not instantiated. So I replaced the line var mysql = require('serverless-mysql'); with:
var mysql = require('serverless-mysql')();
Unfortunately, this results in the following error:
AssertionError: expected [Error: Error: self signed certificate in certificate chain] to be an object
So it seems the connect() method of the real serverless-mysql module is called.
How can I correctly mock serverless-mysql using sinon?

Answer 1

require('serverless-mysql') returns a function that returns a different value each time it is called, so mocking the properties on the result of one call won't affect the returned value of a different call.
That means you have to mock the function itself, which means mocking the entire module.
sinon doesn't provide a way to mock an entire module so you'll have to use something else for that part.
Here is a working test using proxyquire to mock the serverless-mysql module:
const chai = require('chai');
const sinon = require('sinon');
const expect = chai.expect;
const proxyquire = require('proxyquire');
const event = {
pathParameters: {
name: 'johndoe'
}
}
var context;
var successConnectionObject = {
connect: function () {
return Promise.resolve();
},
query: function (sqlQuery, params) {
return Promise.resolve([{
first_name: 'first',
last_name: 'last',
bk: 'bk',
current_team: 'team'
}]);
},
end: function () { }
}
const stub = sinon.stub().returns(successConnectionObject);
const app = proxyquire('../../app.js', { 'serverless-mysql': stub });
describe('Tests', function () {
it('verifies successful response', async () => {
const result = await app.lambdaHandler(event, context);
expect(result).to.be.an('object'); // Success!
expect(result.statusCode).to.equal(200); // Success!
sinon.assert.calledWithExactly(stub, {
config: {
host: process.env.DB_HOST,
user: process.env.DB_USER,
password: process.env.DB_PASSWORD
}
}); // Success!
});
});