nikhil00shinde / promises Goto Github PK

we can see use of callback in 2 ways:

• Events(browser)
• Asynchronous

As long as, we have synchronus task they will execute first than Asynchrounous task.

we have 2 execution state stack and Node API.

• In stack where all synchronous task is done.
• And In node api where all asynchronous task is done.

We have to main problem with the callback

• Callback Hell(Debugging,Readebility & Maintainance issue)
• Inversion of control (control reverts back from you to another, on callback call)

(we have control)

When the task is done, we will get notify and we will do our own operation.

Object

Denote a task (running)

Future value

2 ways :-

• State -> pending and fulfilled
• Result -> undefined and err/data

To understand Promise just, we have to remember that when the task is done we will get notify.

Promise expect a notification on every task to do operation.

• if we give notification then don't do any task then it will give an error.

Only one the reject or resolve function is called, And if we have more than that then the other will get ignore

Three ways to callback:

• ( (resolve,reject)=>{--------})
• (function (resolve,reject){------})
• (laterOp)
  • function laterOp(data) { console.log(data); }

Promise also provide a another good functionality that is it will do asynchronous task in sequenctial manner that is called chaining, the concept is promise chaining.

For a single promise, we can use multiple .then() & .catch() in promise and it will print in the sequencial manner that is what order we write.(Promise.js)

We can use .then() on same promise as again multiple times as CHAIN (Question.js)

• If we don't explicitly return promise inside a .then() then .then() will implicitly return promise in which the resolve value will be undefined
• If we return a value in the .then() function as implicit promise(return) then the chaining .then() get resolve value as return value

.then(), will only run when the resolve is called. If it have more .then() it will wait in queue in Node API then it will run (Promise3.js)

If you apply .then() on resolved promise, it will execute