How do I preload value on variable NodeJS - node.js

Question

I am just learning NodeJS and I come from a Java/Scala background.
I am writing a service that communicates with Amazon SNS and handles endpoints/tokens.
Basically there is a list of SNS applications that I have in my environment, and this list is rarely modified, so I would like to pre-load its values into a variable or constant on server startup.
The SNS-SDK provided by Amazon has this function for listing the applications:
listPlatformApplications(params, callback)
So what I naively tried to do was this:
var applications = [];
var loadApplications = function() {
sns.listPlatformApplications({}, function(err, data){
if (err) {
console.log(err);
} else {
return data['PlatformApplications'].map(function (app) {
return app['PlatformApplicationArn']
});
}
});
}
loadApplications();
And basically what happens is that some calls come in before this callback finishes, when the list is still empty.
How would I go about pre-loading this data, or any other data, before the server starts responding to requests?
Or maybe my reasoning for this is wrong, and there would be another approach to handle this on NodeJS that is more idiomatic

Answer 1

If you absolutely must use a callback instead of a promise (spoiler alert: you don't), you have to call the server startup command within the SNS callback. So you would do something like this:
var startServer = require('my-server.js');
var applications = [];
var loadApplications = function() {
sns.listPlatformApplications({}, function(err, data){
if (err) {
console.log(err);
} else {
var snsData = data['PlatformApplications'].map(function (app) {
return app['PlatformApplicationArn']
});
startServer(snsData)
}
});
}
loadApplications();
== RECOMMENDED SOLUTION ==
If instead, you can use promises (which you absolutely should!), you could start the SNS request at server start and await the result whenever needed. Your code would look something like this:
const startServer = require('my-server.js');
const loadApplications = async () => {
const data = sns.listPlatformApplications({}).promise();
const snsData = data['PlatformApplications'].map(function (app) {
return app['PlatformApplicationArn']
});
return snsData;
};
const applications = loadApplications();
startServer(applications);
// inside my-server.js
const startServer = async (applications) => {
const doSomethingWithApplications = await applications;
...
}