Async

Question 1

What is a Promise, how to create and handle it (example)

Answer 1

• The promise is an object representing the completion or failure of the async operation.

```javascript
function readFilePromise(fileName) {
return new Promise((resolve, reject) => {
fs.readFile(fileName, (err, data) => {
if (err) {
reject(err);
} else {
resolve(data);
}
});
});
}

readFilePromise(“1.txt”)
.then((data) => {
myText += data;
return readFilePromise(“3.txt”);
})
.catch((err) => {
// Catch any rejected promises.
console.log(Error! ${err});
})
.finally(() => {
runFinalCode();
});
~~~

Question 2

List all Promise waiting patterns

Answer 2

Promise.all([ .. ])
- In classic programming terminology, a “gate” is a mechanism that waits on two or more parallel/concurrent tasks to complete before continuing.

Promise.race([ .. ])
- Promise.race is settled as soon as any of the promises you feed it settle, whether they are fulfilled or rejected.

Promise.any([ .. ])
- Promise.any is settled as soon as any of the promises you feed it is fulfilled or they are all rejected, in which case it’s rejected with an AggregateError.

Question 3

Async / await example

Answer 3

async function hello() {
return “Hello”;
}

hello().then(console.log);

// Await must be used in an async function
await hello();

Question 4

What is the output of this code:

```javascript
// Message queue example
{
console.log(“A”);

button.addEventListener(“click”, () => {
// Any other async code
console.log(“B”);
});

console.log(“C”);
}
~~~

Answer 4

Output: A C B

Question 5

How to use a timeout

Answer 5

// Run after at least (!) 2 seconds (when call stack is empty)
let myGreeting = setTimeout(() => {
  alert("Hello, Mr. Universe!");
}, 2000);

Question 6

How to run a task immediately (after current job queue is emptied)

Answer 6

// Run as soon as all of the main thread has finished running
setTimeout(function () {
  alert("World");
}, 0);

// (NodeJS) Similar to setTimeout with 0 seconds.
setImmediate(() => {
  //run something
});

Question 7

How to call a method every 1 second (and how to cancel it)

Answer 7

// Set
const createClock = setInterval(displayTime, 1000);

// Clear
clearInterval(myInterval);

Question 8

How to run code immediatelly (in next tick / animation frame), no delay of a job queue.

Answer 8

// Node
process.nextTick(() => {
  //do something
});

// Browser
function draw(timestamp) {
  // Drawing code goes here
  var rAF = requestAnimationFrame(draw);
}
cancelAnimationFrame(rAF);