Synchronous and Asynchronous Code

Patterns for Dealing with Async Code

Callbacks

function here is the callback function. We pass it to getUser and when it's completed this callback is triggered

getUser(1, function(user) {
    console.log('User', user);
});

Callback function can be in any name but then both parameter and function call must be renamed simultaneously.

function getUser(id, callback) {
   setTimeout(()=> {
       callback({ id:id, username:'test'});
   }, 2000);

Promises

A promise is an object that holds the eventual result of an asynchronous operation

3 states of a promise object

• Pending
• Fullfilled (resolved)
• Rejected (error)

const p = new Promise((resolve, reject) => {
  resolve(1);
//  reject(new Error('message'));
});

   p
     .then(result => console.log('Result : ' + result);
     .catch(err=> console.log('Error : ' + err.message));

Parallel Promises

const p1 = new Promise((resolve) = { 
  setTimeout()=> {
      console.log("Async operation 1");
      resolve(1);
  }, 2000);
});

const p2 = new Promise((resolve) = { 
  setTimeout()=> {
      console.log("Async operation 2");
      resolve(2);
  }, 2000);
});

Promise.all([p1, p2])
  .then(result => console.log(result));
  .catch(err => console.log('Error', err.message));

if any one of the promises rejected all rejected

Promise.race(... if we want to return immediately only one of them resolved

Async/await

async function displayAll(){
const user = await getUser(1);
const repos = await getRepositories(user.id);
}

  displayAll()