I'm seeing the promises and async await in JavaScript. The syntax to generate both if I understand it and I can create them. What I do not understand is because we use async await if the promises are already created to generate asynchronous tasks. I do not understand why async await. Can you help me understand why each one exists and why is it necessary to use both?
Thank you.
The general idea of async/wait is to reduce code and make it less noisy.
Usually with the promises, the method then() is used to wait for the answer:
$http.get(url).then(function(){
//..
});
This translates to:
var response = await $http.get(url);
Imagine now having to wait for an asynchronous process to run another asynchronous process:
$http.get(url).then(function(response){
//...
$http.get(url + response.data.id).then(function(){
console.log(response2.data.message);
});
});
And with async / await it would be:
var response = await $http.get(url);
var response2 = await $http.get(url + response.data.id);
console.log(response2.data.message);
asyn / await also helps avoid the callback hell and makes the code less noisy in the sense that since the code follows a line constant execution, makes it easier to maintain it.
Note: As already mentioned, you can only use await within a function marked as async .
The keywords async/await are to Promise what class is to the classic prototyping of Javascript: sugar syntax .
It's just a different way to create promises without having to use the Promise class directly:
//declaramos una función asincrona
async function devuelveParam(param) {
//el resultado se envolverá dentro de una promesa automáticamente
return param;
}
// Estilo clásico para gestionar una promesa:
let promesa = devuelveParam('Hola');
promesa.then(function (p) {
console.log(p);
});
//con await (ojo, sólo se puede usar await DENTRO de una función async(rona)
async function escribe(param) {
let resultado = await devuelveParam(param);
console.log(resultado);
}
escribe('Adios');