testcallback.js

let cache =[],db=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];
const readline = require("readline");
const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});
rl.on("close", function() {
    console.log("\nBYE BYE !!!");
    process.exit(0);
});
function findFromCacheOrDB(){  
    rl.question("Enter a value to find:", function(find) {
        find=find;
        continueFind(find);
    });
    var continueFind=(find)=>{

        const findInCache= (find)=>{
            return  new Promise(function(resolve, reject) {
                // something that fails
                for (var i in cache){
                    if(cache[i]==find){
                        resolve("found from cache "+find);
                        break;
                    }
                }
                reject("Whoops! couldn't find in cache!");
            });
        }
        const findInDatabase=(find)=>{    
            return  new Promise(function(resolve, reject) {
                // something that succeeds
                for (var i in db){
                    if(db[i]==find){
                        cache.push(db[i]);
                        resolve("found from db");
                        break;
                    }
                }
                reject("Whoops couldn't find in database!");
            });
        }
    
        // find in cache
        findInCache(find).then(function(value) {
            // Use whatever returns fastest
            console.log(value);
            findFromCacheOrDB();
        })
        .catch(function(reason) {
            console.log(reason);
            // if not found in cache then find in database
            findInDatabase(find).then(function(value) {
                // Use whatever returns fastest
                console.log(value);
                findFromCacheOrDB();
            }).catch(function(reason) {
                console.log(reason);
                findFromCacheOrDB();
            });
        });
    }
}
console.log('enter control c to stop');
console.log('enter from a list of '+ db.join(', '));
findFromCacheOrDB();


//  A promise is commonly defined as a proxy for a value that will eventually become available.
//  Promises are one way to deal with asynchronous code, without getting stuck in callback hell.


//  hence this solution is able to solve the latency problem

//  As you can see from the above solution, the promise checks the cache global variable, and if the data is found in it,
//  the promise goes to a resolved state (since the resolve callback was called) and it return to the then block;
//  but if the data is not found in the cache memory it goes to promise2 to check the database,
//  and if the data is found in it 
//  the promise2 goes to resolved state and add the data to cache memory and it return to the then block. 
//  so if the data also is not found in database then it rejects,
//  which means the promise goes to reject state (since the reject callback was called) and it return to catch block