Promises in JavaScript

What is a Promise?

A Promise in JavaScript is used to handle asynchronous operations.
It represents a value that may be available now, later, or never.

A promise has three states:

Pending – The operation is still running.
Fulfilled – The operation completed successfully.
Rejected – The operation failed.

Basic Example

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

  if (success) {
    resolve("Task completed");
  } else {
    reject("Task failed");
  }
});

promise
  .then((message) => console.log(message))
  .catch((error) => console.log(error));

Output:

Task completed

Example with setTimeout

let promise = new Promise((resolve) => {
  setTimeout(() => resolve("Data loaded"), 2000);
});

console.log("Start");

promise.then((result) => console.log(result));

console.log("End");

Output:

Start
End
Data loaded

Promise Chaining

new Promise((resolve) => {
  resolve("Step 1");
})
.then((msg) => {
  console.log(msg);
  return "Step 2";
})
.then((msg) => {
  console.log(msg);
  return "Step 3";
})
.then((msg) => console.log(msg));