NodeJS/MySQL/Promises Troubles - mysql

Question

I'm quite new to NodeJS and JS globally and I'm in trouble while setting and Object Property through a MySQL query.
I'm using Promise to avoid bad asynchronous effect but apparently I'm doing it wrong, the property of my Agent Obejct is never updated.
Here is the code :
class Agent {
constructor(agentId, agentName, agentCountry) {
this.agentId = agentId;
this.agentName = agentName;
this.agentCountry = agentCountry;
}
setAgentCountry () {
var promise = function(agentID) {
return new Promise(function(resolve, reject) {
var query = "SELECT c.CountryID, c.CountryName FROM AgentCountry ac, Country c WHERE ac.AgentID = '" + agentID + "' AND ac.CountryID = c.CountryID";
connection.query(query, function(err, results) {
if (!err) {
resolve(results);
} else {
console.log('Error while performing Query.');
}
});
});
}
promise(this.agentID).then(function(data) {
var string = JSON.stringify(data);
var json = JSON.parse(string);
//the agent property is never updated !!
this.agentCountry = json;
}.bind(this), function(err) {
console.log(err);
});
}
}
I call the method this way :
var agent = new Agent(1,"John Doe", "France");
console.log(agent.agentCountry); //Displays "France"
agent.setAgentCountry();
console.log(agent.agentCountry); //Did not display the table of countries it should
Could you help me with this ?
Thanks

Answer 1

The main problem is that console.log is being executed before the promise being resolved. Writing a console.log inside the "then" clause will show you the timing.
The promise will be resolved or rejected eventually but nobody is waiting for setAgentCountry.

Answer 2

There are several points of order here:
A promise must always be either (1) resolved or (2) rejected. Your error case logs it to the console without calling reject(), so it's stuck in promise limbo for forever when it errors.
Why do you name a variable, promise, the same as the library, Promise?
I think you will find it more modular to just wrap the mysql_conn.query() callback into a promise():
const mysql_conn = mysql.createConnection({
host: mysql_conf.host,
user: mysql_conf.user,
password: mysql_conf.password
});
mysql_conn.queryPromiser = function(sql, args) {
return new Promise(function(resolve, reject) {
mysql_conn.query(
sql,
args,
function(err, results, fields) {
if (err) {
reject(err);
} else {
resolve( {"results": results, "fields": fields} );
}
}
);
});
};
then you can use it like so:
class Agent {
constructor(agentId, agentName) {
this.agentId = agentId;
this.agentName = agentName;
this.agentCountry = null;
}
configureCountryPromiser() {
var sql = "SELECT country FROM agent_countries WHERE agent_id = ?";
var args = [ this.agentId ];
var that = this;
return mysql_conn.queryPromiser(sql, args)
.then(function(data) {
if (data.results.length) {
that.agentCountry = data.results[0].country;
} else {
// handle case where agent_id is not found in agent_countries
}
});
}
};
agent_instance = new Agent(1, "Benoit Duprat");
agent_instance.configureCountryPromiser()
.then(function() {
console.log("agent country configured to ", agent_instance.agentCountry);
}).catch(console.error);
Please note that I have not tested the class code, but the general idea should be enough to answer your question.