What is a Promise in JavaScript, and what are its parameters?

A Promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It is used to handle asynchronous operations more efficiently, avoiding callback hell and making the code more readable.

A promise can be in one of three states:

1. Pending – Initial state, neither fulfilled nor rejected.
2. Fulfilled – The operation was successful, and the promise has a result.
3. Rejected – The operation failed, and the promise has a reason for the failure.

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Syntax:

let promise = new Promise(function(resolve, reject) {
  // Asynchronous task here...

  if (/* task successful */) {
    resolve('Success Result');
  } else {
    reject('Error Reason');
  }
});

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Parameters of a Promise:

The Promise constructor takes a single argument, which is a callback function:

new Promise(function(resolve, reject) { ... })

The callback function itself takes two parameters:

1. resolve(value) → A function that is called when the operation is successful.
2. reject(reason) → A function that is called when the operation fails.

These parameters are provided by JavaScript, and you use them inside the promise body.

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Example:

let promise = new Promise(function(resolve, reject) {
  let success = true;

  setTimeout(() => {
    if (success) {
      resolve('Task completed successfully');
    } else {
      reject('Task failed');
    }
  }, 1000);
});

promise
  .then(result => console.log(result))  // Handles success
  .catch(error => console.error(error)) // Handles failure
  .finally(() => console.log('Task completed (either success or failure)'));

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Key Promise Methods:

Method	Description
.then()	Handles the fulfilled state and receives the result.
.catch()	Handles the rejected state and receives the error reason.
.finally()	Executes code after promise is settled (fulfilled/rejected).

Using Promises simplifies asynchronous code, improving readability and reducing the nesting issues associated with callbacks.