Skip to content

Creating Promises

Creating Promises

Introduction

You create a Promise using the new Promise(executor) constructor, where the executor function receives resolve and reject callbacks.

Basic Creation

const promise = new Promise((resolve, reject) => {
  // Async operation here
  setTimeout(() => {
    const success = true;

    if (success) {
      resolve('Operation succeeded!');
    } else {
      reject(new Error('Operation failed'));
    }
  }, 1000);
});

Consuming a Promise

promise
  .then(result => {
    console.log(result); // 'Operation succeeded!'
  })
  .catch(error => {
    console.error(error);
  })
  .finally(() => {
    console.log('Always runs');
  });

Converting Callback to Promise

function readFilePromise(path) {
  return new Promise((resolve, reject) => {
    readFile(path, (err, data) => {
      if (err) reject(err);
      else resolve(data);
    });
  });
}

readFilePromise('/path/to/file')
  .then(data => console.log(data))
  .catch(err => console.error(err));

Summary

new Promise(executor) creates a new promise
Executor runs immediately (synchronously)
resolve(value) transitions to fulfilled
reject(error) transitions to rejected
.then(), .catch(), .finally() consume the result

Further Reading

MDN: Using Promises