How to ensure that one function completes before other function - node.js

Question

I am fairly new to Node.js, and what I am trying to achieve is to have two separate functions. One for Auth and one for sending data (So that I don't run into rate login limits if I were to simply use a callback after conn.login finishes). I tried to set this up in node like this:
var _request = {
url: '/services/data/v45.0/actions/custom/flow/Test1',
method: 'POST',
body: JSON.stringify({
"inputs": [{}]
}),
headers: {
"Content-Type": "application/json"
}
};
var conn = new jsforce.Connection({
clientId: process.env.cliendId,
clientSecret: process.env.clientSecret,
version: "45.0"
});
function sfdcAuth() {
conn.login(process.env.sfdcUser, process.env.sfdcUserPass, (err, userInfo) => {
if (err) {
console.log(err)
}
conn = conn;
console.log("Done")
});
}
function sfdcQuery() {
conn.request(_request, function(err, resp) {
console.log(resp);
console.log(err)
});
}
sfdcAuth()
sfdcQuery()
But because js is asynchronous it runs the second function without waiting for the first function to finish.

Answer 1

The simplest way is to pass your second function as a callback to your first function, which it can call when it’s done:
function sfdcAuth(callback) {
conn.login(process.env.sfdcUser, process.env.sfdcUserPass, (err, userInfo) => {
if (err) {
console.log(err);
}
// Invoke callback when done
callback();
});
}
function sfdcQuery() {
conn.request(_request, function(err, resp) {
console.log(resp);
console.log(err);
});
}
// Pass second function as callback to the first
sfdcAuth(sfdcQuery);
You could also make use of promises:
function sfdcAuth(callback) {
return new Promise((resolve, reject) => {
conn.login(process.env.sfdcUser, process.env.sfdcUserPass, (err, userInfo) => {
if (err) {
reject(err);
}
resolve(userInfo);
});
});
}
function sfdcQuery() {
return new Promise((resolve, reject) => {
conn.request(_request, function(err, resp) {
if (err) {
reject(err);
}
resolve(resp);
});
});
}
// Wait for promise to resolve before invoking second function
sfdcAuth()
.then(result => {
// Do something with result
return sfdcQuery();
})
.then(result => {
// You can continue the chain with
// the result from "sfdcQuery" if you want
})
.catch(err => {
// Handle error
});