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Promise States

Promise States

Introduction

A Promise has three states: pending, fulfilled, and rejected. Once a promise settles (fulfilled or rejected), it cannot change state.

Visual Explanation

flowchart TD
    A["new Promise(executor)"]
    B["⏳ Pending"]
    C["✅ Fulfilled (resolve)"]
    D["❌ Rejected (reject)"]

    A --> B
    B --> C
    B --> D
    C --> E[".then()"]
    D --> F[".catch()"]
    E --> G[".finally()"]
    F --> G

State Transitions

const promise = new Promise((resolve, reject) => {
  // Promise starts as PENDING

  // After resolve() → FULFILLED
  resolve('Success');

  // After reject() → REJECTED
  // reject(new Error('Failed'));

  // Once settled, further resolve/reject calls are ignored
  resolve('This is ignored'); // Already resolved above
});

console.log(promise); // Promise { 'Success' }

Checking State

// You can't directly check a promise's state from user code
// But you can observe it through callbacks:

const p = new Promise((resolve) => {
  setTimeout(() => resolve('done'), 1000);
});

console.log(p); // Promise { <pending> } — still pending

p.then(value => {
  console.log(value); // 'done' — now fulfilled
});

Summary

Pending: Initial state, not yet settled
Fulfilled: Operation completed successfully (resolve called)
Rejected: Operation failed (reject called)
Once settled, state is immutable

Further Reading

MDN: Promise