• Promise is a special JavaScript object, and it can act as a placeholder.
• Promise is a placeholder for a certain period of time until we receive a value from the async operation.
• It is a container for future value.
• MDN definition: the promise is an object representing the eventual completion or failure of an async operation.
• Initially promise object can be an undefined value. After completion of the async operation, it will be replaced with new data.
• It is immutable.
• If you pass the data horizontally that time you can use the return keyword; otherwise, you lose data in the chain.

You need to know two things about promises :

1. How to create a promise.
2. How to consume a promise.

The Promise object has 4 major properties:

1. status.
2. value.
3. onFulfillment.
4. onReject.

Status or state:

• Status shows the current promise state.
• Initially, the status is pending. After some time, the status gets updated.
• promise may have one of three states:

1. Pending state: initial state
2. fulfilled state: meaning that the operation was completed successfully. 3). rejected state: meaning that the operation failed. Note: One of these three states will be there.

Value: When the status of the promise is pending, then this property is undefined. The moment the promise is resolved, the value property is updated from undefined to the new value. So the value property acts like a placeholder till the time the promise finishes.

OnFulfillment: This is an array that contains functions that we attach to our promise object. We can attach some functions using the .then() method. When the value property is updated from undefined to the new value, JavaScript gives the chance to those attached functions one by one with the value property as their argument(if there is no piece of code in the call stack or global code).

Steps to create a promise : 1 - To create a promise, call the promise constructor. 2 - the promise constructor takes a callback as an argument. 3 - the callback function, which is passed inside the constructor function, accepts arguments such as resolve and reject. 4 - Inside the callback function, you can write your own logic. 5 - If you want to return something on success, then you can call the resolve function with data. 6 - error: call reject method with some error.