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Capture MySQL query result as var [js]

I'm currently trying to build a discord bot, and I want to use a database for some aspects of it. Currently, I'm trying to add a command that would return the names of all the tables I have in the database, and for the most part I have it down.
The part that I'm struggling with is actually getting the names back out as a var. Every guide and stackoverflow question that I've been able to find on it assume that you just want to get that result and then print it to the console, but I need to return it back to the method that called this.
My previous attempt was setting an outside variable and using a promise to wait for it to change, but I couldn't get that to work, most likely because I don't fully understand how Promises work. My current attempt uses setTimeout() to check, but that just returns the asyncID of either the first or second iteration.
Any help either in making either of these work or completely scrapping them and doing this a different way is very welcome.
Previous code:
function listTables() {
db.query('SELECT table_name FROM information_schema.tables WHERE table_schema=\'' + dbName + '\'', (error, results) => {
if(error) throw error;
let temp = '';
results.forEach((item) => {
temp += item.table_name + ', ';
}); temp = temp.slice(0, -2);
setReturn(temp);
});
let out = checkReturn().then((value) => {
return value();
}).catch((error) => {
console.log(error);
return '';
});
returnValue = null;
return out;
}
var returnValue = null;
function setReturn(value) {
returnValue = value;
}
async function checkReturn() {
console.log('Checking Return: ' + returnValue);
let promise = new Promise((resolve, reject) => {
if(returnValue === null) reject('Var not set');
else resolve(returnValue)
});
return await promise;
}
Current Code:
function listTables() {
setReturn(null);
db.query('SELECT table_name FROM information_schema.tables WHERE table_schema=\'' + dbName + '\'', (error, results) => {
if(error) throw error;
let temp = '';
results.forEach((item) => {
temp += item.table_name + ', ';
}); temp = temp.slice(0, -2);
setReturn(temp);
});
return checkReturn();
}
var returnValue = null;
function setReturn(value) {
returnValue = value;
}
function checkReturn() {
console.log('Checking Return: ' + returnValue);
if(returnValue === null) {
return setTimeout(checkReturn, 50);
} else {
return returnValue;
}
}

You need to modify the listTables function to return a promise.
function listTables() {
return new Promise((resolve, reject) => {
db.query('SELECT table_name FROM information_schema.tables WHERE table_schema=\'' + dbName + '\'', (error, results) => {
if(error) {
reject(error);
return;
}
let temp = '';
results.forEach((item) => {
temp += item.table_name + ', ';
}); temp = temp.slice(0, -2);
resolve(temp);
});
});
}
// Usage of `listTables()`
listTables()
.then(result -> {
// Do process result
});

Need JavaScript example for fetch() etc

I want to learn such new JavaScript features as fetch() and arrow functions. To this end, I selected a function from a recent app, and attempted to replace older features with new. Very little success. Here's my original function:
function popNames(arNumbers,ctrlName) {
var arSortedList = [];
var strNameList = "";
$.getJSON("NAME.json").done(function(zdata) {
$.each(arNumbers, function(i, ydata) {
$.each(zdata.NAME, function(k,v) {
if(v.idName == ydata) {// important: === did NOT work
if(ctrlName) arSortedList.push(v.last + ", " + v.first + ";" + v.idName);
else arSortedList.push(v.last + ", " + v.first);
}
}); // each element of NAME.json
}); // each idName value in the array passed
if(ctrlName) {
setOptions(arSortedList, ctrlName);
} else {
strNameList = arSortedList.join();
}
}); // getJSON NAME
}
I was successful using this line:
fetch("NAME.json").then(zdata => zdata.json())
but nothing I did after that worked. I'd appreciate seeing an example from which I can learn.

function popNames(arNumbers,ctrlName) {
let arSortedList = [];
let strNameList = "";
fetch("NAME.json").then(zdata => zdata.json())
.then(zdata => {
for(const ydata of arNumbers) {
for(const v of zdata.NAME) {
if(v.idName == ydata) { // important: === did NOT work
if(ctrlName) arSortedList.push(v.last + ", " + v.first + ";" + v.idName);
else arSortedList.push(v.last + ", " + v.first);
}
}
}
if(ctrlName) {
setOptions(arSortedList, ctrlName);
} else {
strNameList = arSortedList.join();
}
}); // getJSON NAME
}
I was researching why I couldn't next two Array.forEach statements, and discovered a new iterable construction (for...of).

ReplaceAll causing issues in array.reduce

I am still pretty new to this, so forgive me if I dont' say this correctly. We have an array.reduce that calls a method with a returning promise that iterates through a list of files and post results to the db. Everything was working great, until it ran into a field that had an apostrophe in it and then the db insert fails. This is the field value. 'Expected 100002822' to be 100002822.'
I tried adding a replaceAll on the field and now get an error in the array.reduce.
Here is the .reduce
console.log('Found test results in ' + matches.length + ' files. Parsing and posting to the database now...');
var startTime = moment();
var parser = new Parser();
matches.reduce(function (p, val) {
return p.then(function () {
return parser.parseResults(val);
});
}, Promise.resolve()).then(function (finalResult) {
var endTime = moment();
var testDuration = moment.duration(endTime.diff(startTime));
console.log(chalk.blue('*** File parsing time: ' + testDuration.humanize() + ' ***'));
if (finalResult.insertSuccess == matches.length) {
var publishOut = {
totalFiles: matches.length,
totalTests: 0,
totalTestsSuccess: 0,
totalTestsFailed: 0
}
publishOut.totalTests += finalResult.totalTests;
publishOut.totalTestsSuccess += finalResult.testPassedCount;
publishOut.totalTestsFailed += finalResult.testFailedCount;
console.log(`Successfully inserted ${finalResult.insertSuccess} of ${publishOut.totalTests} test results.`);
// for (var i = 0; i < matches.length; i++) {
// var currentFile = `./testing/results/${matches[i]}`;
// fs.unlinkSync(currentFile);
// }
resolve(publishOut);
} else {
reject('Only ' + finalResult.insertSuccess + ' of ' + matches.length + ' successfully posted to the database');
}
}, function (err) {
reject('error in reduce', err);
});
I have tried several different ways of using the replaceAll with the same failure. It hits this code from the array.reduce
}, function (err) {
reject('error in reduce', err);
});
And this is the called method. The added code causing the failure in the .reduce is this Message = expectation.message.replaceAll("'", "");
protractorParser.prototype.parseResults = function (fileName) {
return new Promise((resolve, reject) => {
//console.log('In parseresults', fileName);
var currentFile = './testing/results/' + fileName
json.readFile(currentFile, function (err, obj) {
if (err != null) {
console.log('error reading file', err);
reject(err);
} else {
resolve(obj);
}
});
}).then(function (obj) {
var results = [];
for (var suite in obj) {
var specs = obj[suite].specs;
for (let i = 0; i < specs.length; i++) {
const assert = specs[i];
const tcR = /TC[\d]+/;
const tc = assert.description.match(tcR);
let Passed = 1;
let Message = '';
let Stack = '';
testResults.totalTests++;
if (assert.failedExpectations.length) {
const expectation = assert.failedExpectations[assert.failedExpectations.length - 1];
Passed = 0;
Message = expectation.message.replaceAll("'", "");
Stack = expectation.stack.split('\n')[1].trim();
testResults.testFailedCount++
} else {
testResults.testPassedCount++
}
if (tc != null) {
const time = moment().utcOffset(config.get('settings.timeOffset')).format('YYYY-MM-DDTHH:mm:ss');
const promise = utility.TestDataManager.insertAutomationResults(tc[0], assert.description, Passed, process.env.testBuild, 'P', Message, Stack, 0, time, '');
results.push(promise.then(() => {
//fs.unlinkSync(currentFile);
testResults.insertSuccess++;
//console.log('insertSuccess', testResults.insertSuccess);
},
err => { console.log('… failed', err); throw err; }
));
} else {
console.log('no test case found for test: ' + assert.description + ' -- skipping');
// I don't think you want to `throw err` here, right?
}
}
}
return Promise.all(results).then(() => testResults);
});
};

Trouble with Node JS promises

I cannot find a solution to why this function returns before my message array is updated with the necessary values.
var calculateDistance = function (message, cLongitude, cLatitude, cSessionID) {
return new Promise(function (resolve, reject) {
distance.key = options.apiKey;
distance.units('metric');
var origins = [];
origins.push(cLatitude + ',' + cLongitude);
message.forEach(function (obj) {
obj.sessionId = cSessionID;
var destinations = [];
destinations.push(obj.geoLocation.latitude + ',' + obj.geoLocation.longitude);
distance.matrix(origins, destinations, function (err, distances) {
if (err) {
return console.log(err);
}
if (!distances) {
return console.log('no distances');
}
if (distances.status == 'OK') {
for (var i = 0; i < origins.length; i++) {
for (var j = 0; j < destinations.length; j++) {
var origin = distances.origin_addresses[i];
var destination = distances.destination_addresses[j];
if (distances.rows[0].elements[j].status == 'OK') {
var distance = distances.rows[i].elements[j].distance.text;
console.log('Distance from ' + origin + ' to ' + destination + ' is ' + distance);
obj.distance = distance;
} else {
console.log(destination + ' is not reachable by land from ' + origin);
obj.distance = 'N/A';
}
}
}
}
});
});
return resolve(message);
});
}
Could someone point out to me what i am doing wrong here.
Regards
Jimmy

var async = require('async');
var calculateDistance = function (message, cLongitude, cLatitude, cSessionID) {
return new Promise(function (resolve, reject) {
distance.key = options.apiKey;
distance.units('metric');
var origins = [];
origins.push(cLatitude + ',' + cLongitude);
async.each(message, function(obj, callback) {
obj.sessionId = cSessionID;
var destinations = [];
destinations.push(obj.geoLocation.latitude + ',' + obj.geoLocation.longitude);
distance.matrix(origins, destinations, function (err, distances) {
if (err) {
callback(err);
}
if (!distances) {
callback('no distances');
}
if (distances.status == 'OK') {
for (var i = 0; i < origins.length; i++) {
for (var j = 0; j < destinations.length; j++) {
var origin = distances.origin_addresses[i];
var destination = distances.destination_addresses[j];
if (distances.rows[0].elements[j].status == 'OK') {
var distance = distances.rows[i].elements[j].distance.text;
console.log('Distance from ' + origin + ' to ' + destination + ' is ' + distance);
obj.distance = distance;
} else {
console.log(destination + ' is not reachable by land from ' + origin);
obj.distance = 'N/A';
}
}
}
callback(null);
}
});
},function(err){
if(err){
return reject(err);
}else{
return resolve(message);
}
});
});
};

This is happening because your distance.matrix(origins, destinations, callback )is asynchronous . In above code distance.matrix method is getting pushed to event loop and continues it's execution and before that method callbacks gets executed resolve(message) is returned .

You need to read up on promises. It looks to me as if you are thinking of promises as magical way to set up a callback. "Magic" tends to mean "something I don't need to understand." In this case that's not true.
That executor function of yours (that is the function which begins with 'function(resolve,reject)') should set up one asynchronous request. If, as is normal, the request has a callback you put the 'resolve' and 'reject' in the callback. The result will be a promise object which has methods 'then' and 'catch' where your post-request processing goes.
Since you want to fill up a matrix with the results of a lot of async requests, you will need to read about 'Promise.all' so you can react when all of them have resolved.

Node.js Function Flow

When I get a request, I want it to generate a 4-character code, then check if it already exists in the database. If it does, then generate a new code. If not, add it and move on. This is what I have so far:
var code = "";
var codeFree = false;
while (! codeFree) {
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var code = "";
for (var i = 0; i < 4; i++) {
var rand = Math.floor(Math.random() * chars.length);
console.log(rand);
code += chars.charAt(rand);
}
console.log("Code: %s generated.", code);
client.execute("select * from codes where code=" + code, function(err, result) {
if (! err) {
if (result.rows.length > 0) {
codeFree = false;
} else {
codeFree = true;
}
} else {
console.log('DB ERR: %s', err);
}
console.log(codeFree);
});
console.log('here');
}
This does not do nearly what I want it to do. How can I handle something like this?

You are doing an async task.
When you have an asyncronous task inside your procedure, you need to have a callback function which is going to be called with the desired value as its argument.
When you found the free code, you call the function and passing the code as its argument, otherwise, you call the getFreeCode function again and passing the same callback to it. Although you might consider cases when an error happens. If your the db call fails, your callback would never get called. It is better to use a throw/catch mechanism or passing another argument for error to your callback.
You can achieve what you need to do by doing it this way:
function getFreeCode(callback) {
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var code = "";
for (var i = 0; i < 4; i++) {
var rand = Math.floor(Math.random() * chars.length);
console.log(rand);
code += chars.charAt(rand);
}
console.log("Code: %s generated.", code);
client.execute("select * from codes where code="+code, function(err, result) {
if(!err) {
if(result.rows.length > 0) {
getFreeCode(callback);
} else {
callback(code);
}
}else {
console.log('DB ERR: %s', err);
}
console.log(codeFree);
});
console.log('here');
}
// in your main:
getFreeCode(function (code) {
console.log(' this code was free: ' + code)
})

I recommend you look into two alternatives to help deal with asynchronous code.
node generator functions using the 'yield' keyword
promises
Using generators requires running a recent version of node with the --harmony flag. The reason I recommend generators is because you can write code that flows the way you expect.
var x = yield asyncFunction();
console.log('x = ' + x);
The previous code will get the value of x before logging x.
Without yielding the console.log would write out x before the async function was finished getting the value for x.
Your code could look like this with generators:
var client = {
execute: function (query) {
var timesRan = 0;
var result = [];
return function () {
return setTimeout(function () {
result = ++timesRan < 4 ? ['length_will_be_1'] : [];
return result;
},1);
};
}
};
function* checkCode () {
var code;
var codeFree = false;
while(!codeFree) {
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
code = "";
for (var i = 0; i < 4; i++) {
var rand = Math.floor(Math.random() * chars.length);
console.log(rand);
code += chars.charAt(rand);
}
console.log("Code: %s generated.", code);
try {
var result = yield client.execute("select * from codes where code="+code);
codeFree = result.rows.length > 0 ? false : true;
}catch(e) {
console.log('DB ERR: %s', err);
} finally {
console.log(codeFree);
}
console.log('here');
}
}
checkCode().next();
You would leave off the client object. I only added that to make a working example that fakes an async call.
If you have to use an older version of node or do not like the yield syntax then promises could be a worthy option.
There are many promise libraries. The reason I recommend promises is that you can write code that flows the way you expect:
asyncGetX()
.then(function (x) {
console.log('x: ' + x);
});
The previous code will get the value of x before logging x.
It also lets you chain async functions and runs them in order:
asyncFunction1()
.then(function (result) {
return asyncFunction2(result)
})
.then(function (x) { /* <-- x is the return value from asyncFunction2 which used the result value of asyncFunction1 */
console.log('x: ' + x);
});
Your code could look like this with the 'q' promise library:
var Q = require('q');
var client = {
timesRan: 0,
execute: function (query, callback) {
var self = this;
var result = {};
setTimeout(function () {
console.log('self.timesRan: ' + self.timesRan);
result.rows = ++self.timesRan < 4 ? ['length = 1'] : [];
callback(null, result);
},1);
}
};
function checkCode () {
var deferred = Q.defer();
var codeFree = false;
var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
var code = "";
for (var i = 0; i < 4; i++) {
var rand = Math.floor(Math.random() * chars.length);
console.log('rand: %s', rand);
code += chars.charAt(rand);
}
console.log("Code: %s generated.", code);
client.execute("select * from codes where code="+code, function(err, result) {
console.log('err: '+err+', result: ' + JSON.stringify(result));
console.log('result.rows.length: ' + result.rows.length);
if(!err) {
if(result.rows.length > 0) {
codeFree = false;
console.log('result.rows: %s, codeFree: %s', result.rows, codeFree);
checkCode();
} else {
codeFree = true;
console.log('line 36: codeFree: ' + codeFree);
deferred.resolve(code);
}
}else {
console.log('DB ERR: %s', err);
deferred.reject(err);
}
console.log(codeFree);
});
console.log('waiting for promise');
return deferred.promise;
}
checkCode()
.then(function (code) {
console.log('success with code: ' + code);
})
.fail(function(err) {
console.log('failure, err: ' + err);
});
Also omit the client object here. I only added that to make a working example that fakes an async call.
Promises and generators definitely take some time to get used to. It's worth it because they make the code a lot easier to follow in the end than code written with nested callbacks.