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Consuming Promises

Consuming Promises

Introduction

You consume a promise using .then() for success, .catch() for errors, and .finally() for cleanup that runs regardless of outcome.

Basic Usage

const promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve('Data loaded'), 1000);
});

promise
  .then(result => {
    console.log(result); // 'Data loaded'
  })
  .catch(error => {
    console.error(error);
  })
  .finally(() => {
    console.log('Cleanup — always runs');
  });

then() Returns a New Promise

Promise.resolve(5)
  .then(value => value * 2)      // Returns 10 (wrapped in Promise)
  .then(value => value + 3)      // Returns 13
  .then(value => console.log(value)); // Logs 13

Error Handling

Promise.resolve(5)
  .then(n => { throw new Error('Failed!'); })
  .then(n => n * 2)              // Skipped (error)
  .catch(err => {
    console.log(err.message);    // 'Failed!'
    return 0;                    // Recovery value
  })
  .then(n => console.log(n));    // 0

Summary

.then(onFulfilled) handles success
.catch(onRejected) handles errors
.finally(onFinally) runs regardless (no arguments)
Each .then() returns a new promise for chaining
Errors skip to the nearest .catch()

Further Reading

MDN: Promise.then