Mocking/Stubbing/unit testing mysql streaming query rows node js - mysql

Question

I have a following function which uses streaming-query-rows of mysql node js module. How can i unit test the below function and also i want to mock the database behavior instead of connecting to database while unit test.
'processRow' and ''wirteCsvFile'' function both are synchronous task.
function executeTask(sql_connection,sql_query) {
let query = sql_connection.query(sql_query);
let showInfo = {};
let showids = [];
query
.on('error', (error) => {
console.error(`error executing query --> ${error}`);
})
.on('result', function (row) {
sql_connection.pause();
processRow(row, showInfo, showids, function () {
sql_connection.resume();
});
})
.on('end', function () {
showids.forEach(showid => {
if (showInfo[showid].faults.length === 0) {
delete showInfo[showid];
}
});
wirteCsvFile(showInfo, (error, done) => {
if (error) {
console.error(error);
} else {
console.log("done");
process.exit();
}
})
});
}

Answer 1

You can stub the query function to return whatever you want instead of making request to database:
sinon.stub(connection, "query").callsFake(() => /* whatever you want here */);
You should also break executeTask into smaller functions, for ex:
function errorHandler(error) {
console.error(`error executing query --> ${error}`);
}
function resultHandler(data, row) {
sql_connection.pause();
processRow(row, data.showInfo, data.showids, function() {
sql_connection.resume();
});
}
function endHandler(data) {
data.showids.forEach(showid => {
if (data.showInfo[showid].faults.length === 0) {
delete data.showInfo[showid];
}
});
wirteCsvFile(data.showInfo, (error, done) => {
if (error) {
console.error(error);
} else {
console.log("done");
process.exit();
}
})
}
function executeTask(sql_connection, sql_query) {
let query = sql_connection.query(sql_query);
let data = {
showInfo: {},
showids: [],
};
query.on('error', errorHandler)
.on('result', resultHandler.bind(null, data))
.on('end', endHandler.bind(null, data));
}
Now you can test errorHandler, resultHandler, endHandler separately

Answer 2

What I'm thinking is we can mock the sql_connection with a class of Event Emitter.
const sinon = require("sinon");
const assert = require('assert');
const EventEmitter = require('events');
const src = require('....'); // your source file that contain `executeTask`
// Create mock emitter
class QueryEmitter extends EventEmitter {}
describe('test execute task', function() {
const queryEmitter = new QueryEmitter();
// we build mock connection that contains all methods used as in `sql_connection`
const conn = {
query: sinon.stub().returns(queryEmitter),
pause: sinon.spy(),
resume: sinon.spy()
};
const query = 'SELECT *';
before(function() {
src.executeTask(conn, query);
});
it('calls query', function() {
assert(conn.query.calledWith(query));
});
it('on result', function() {
queryEmitter.emit('result');
assert(conn.pause.called);
// assert if processRow is called with correct arguments
// assert if conn.resume is called
});
it('on end', function() {
queryEmitter.emit('end');
// assert if writeCsvFile is called
});
// probably is not needed since you only call console.log here
it('on error', function() {
queryEmitter.emit('error');
});
});
Hope it helps