Waiting for a callback - node.js

Question

I'm working with callbacks to improve my program's efficiency. I would like to wait for my 'a' variable to get the value from callback if it hasn't already at some point. My code looks like this:
function myFunction(callback){
request("url", function(error, response, body) {
if (!error && response.statusCode == 200)
{
result = JSON.stringify(JSON.parse(body));
return callback(null, result);
}
else
{
return callback(error, null);
}
});
}
var a = myFunction(function(err, data){
if(!err)
{
return(data);
}
else
{
return(err);
}
});
//A CHUNK OF CODE EXECUTES HERE
//I'D LIKE TO CHECK IF THE a VARIABLE GOT THE VALUE (if the callback was executed), note that I don't want to nest the chunk inside the callback, but rather I'd like to check that outside function callback via IF statement/loop or some similar alternative
//MORE CODE CONNECTED TO THE CALLBACK
Any ideas on how I can wait for the variable to get the value if it didn't get it already by the time that the chunk of code is executed? I know that the sole intention of using callbacks is to not wait but in this instance it is necessary for me so just don't bother being annoying please :)

Answer 1

I'd suggest trying async/await and the request-promise-native module, this keeps the syntax very simple. If the call fails for some reason an error will be thrown, this is easy to catch:
For example:
const rp = require('request-promise-native');
async function testAsyncRequest() {
try {
let promise = rp('https://my-json-server.typicode.com/typicode/demo/posts');
// Do some stuff here...
let result = await promise;
console.log("Result: ", result);
} catch (err) {
console.error(`Something went wrong: ${err.message}`);
}
}
testAsyncRequest();

Answer 2

So I've ended up making a little compromise (had to nest), but the code works as intended:
var something;
function myFunction(callback){
request("url", function(error, response, body) {
if (!error && response.statusCode == 200)
{
result = JSON.stringify(JSON.parse(body));
return callback(null, result);
}
else
{
return callback(error, null);
}
});
}
var a = myFunction(function(err, data){
if(!err)
{
something = data;
return(data);
}
else
{
return(err);
}
});
//A CHUNK OF CODE EXECUTES HERE
var something_cachedValue=something;
doStuff();
function doStuff() {
if(something===something_cachedValue) {
setTimeout(doStuff, 10);
return;
}
something_cachedValue = something;
//MORE CODE CONNECTED TO THE CALLBACK
}
So basically I'm checking if the callback has completed every 10ms by re-calling the function. When it's completed, the "//MORE CODE CONNECTED TO THE CALLBACK" executes. Not the cleanest and most fancy way to do it but it does the job.