sample3.js

// Sample Javascript Program, Promises and Async Await

// Use of too many callbacks results in code which is not easily understandable. Hence promises are used instead of callbacks.
// A promise is like a promise in real life, it can either be fulfilled or resolved, or not fulfilled and rejected.


// Promises - Section One
function checkIfEven(num) {
    return new Promise( (resolve, reject) => { // This returns a new instance of promise, passed a callback function to it with resolve and reject as params
        if(num%2==0) // Check if number is even, if yes, resolve the promise and send back the number.
            resolve(num);
        else // If odd, reject and send back a string "Odd".
            reject("Odd");
    });
}

checkIfEven(133)
    .then( (number) => { // Check if 133 is even, then execute nameless function: (num) => {}
        console.log(`${number} is Even`); // Value of number would've been the sent back by checkIfEven(...), had it been even
    })
    .catch( (errMessage) => {
        console.log(`${errMessage} number found`); // Value of errMessage is "Odd", because 133 is not even
    });

// // Promises - Section Two

// // Promise.all can be used if there are many promises that need to be satisfied.

// function checkIfEven(num) {
//     return new Promise( (resolve, reject) => { // This returns a new instance of promise, passed a callback function to it with resolve and reject as params
//         if(num%2==0) // Check if number is even, if yes, resolve the promise and send back the number.
//             resolve(num);
//         else // If odd, reject and send back a string "Odd".
//             reject("Odd");
//     });
// }

// function checkIfGreaterThanTen (num) {
//     return new Promise ( (resolve, reject) => {
//         if(num>10)
//             resolve(num);
//         else
//             reject("Less than 10");
//     });
// }

// var promises = []
// promises.push (checkIfEven(122));
// promises.push (checkIfGreaterThanTen(114));

// Promise.all(promises)
//     .then( (num) => { // Num is an array containing 122 and 114, the first promise returns 112, the second promise returns 114
//         console.log(num);
//     })
//     .catch( (errMessage) => {
//         console.log(errMessage); // errMessage contains the error which occurs first in the promises (if any)
//     });

// // Async Await - Section Three

// // Async Await implements promises, but makes the code look better, removing the .then()s and the .catch()s

// function returnAfter2s (num) { // Function which returns a value after two seconds
//     return new Promise( (resolve, reject) => {
//         setTimeout ( () => {
//             resolve(num);
//         }, 2000);
//     })
// }

// var n = returnAfter2s(3);
// console.log(n); // Prints Promise { <pending> }, because this gets executed before the value was returned to n.

// returnAfter2s(32).then( (n) => { // Implement with promises
//     console.log(n); // Prints 32 after promise is resolved
// })

// async function display () {
//     var n = await returnAfter2s(23); // waits till the promise is resolved and stores the resolved value in n.
//     console.log(n); // After the promise is resolved an n obtains a value, prints n
// }

// display(); // call function display

// // Note: The keyword await can only be used inside a function specified as async.