Promises

• To handle asynchronus operations
• Efficient way instead of multiple complex call back functions which makes code very large and unmanagable.
• Makes code look simple and easy to understand.

Promises states

States	Description
Rejected	If the action specified in the promise is failed
Fulfilled	If the action specified in the promise is fulfilled
Settled	Either it is fulfilled or rejected
Pending	If the action specified in the promise is still processing

Syntax

let promise = new Promise(function(resolve, reject){
    //code
}); 

Note

• If the action specified in the code of Promise is fulfilled, then the result will be present in resolve(value) which is again a call back function.
• If the action specified in the code of Promise is rejected, then the error details will be in reject(error)

Example

let promise = new Promise(function(resolve, reject) { 
let x = 3; 
let y = 9;
let val= x*y;

if(val%2 == 0) { 
	resolve(); 
} else { 
	reject(); 
} 
}); 

promise. 
	then(function () { 
		console.log('Result is an even number'); 
	}). 
	catch(function () { 
		console.log('Rejected'); 
	}); 

Run here

Async-Await

• Async-Await are the more cleaner and efficient way of writing asynchronous operations in JS.
• Efficient way instead of multiple complex promises which makes code very large and unmanagable.
• Makes code look simple and easy to understand.
• Async functions always return a value, either returns value if the promise is fulfilled or error if the promise is rejected.

Syntax

async function functioname(parameters){
	//code
}

Example

async function getTodos(userObj){
	const res = await fetch([url]);
	const data = await res.json()    
	return data;
}

let data = await getTodos({fn: "foo"});

Fetching API data using both promise method and async and await method.

• promise

const url = 'https://restcountries.com/v2/all'
fetch(url)
  .then(response => response.json())
  .then(data => {
    console.log(data)
  })
  .catch(error => console.error(error))

• async and await

const fetchData = async () => {
  try {
    const response = await fetch(url)
    const countries = await response.json()
    console.log(countries)
  } catch (err) {
    console.error(err)
  }
}
console.log('===== async and await')
fetchData()