Promises — then, catch & finally

A Promise represents work that finishes later (API call, file read, timer). ES6 standardized Promises. Node.js and browsers use them everywhere — async/await (next lesson) is built on top.

Three states

• Pending — still running
• Fulfilled — success, .then() runs
• Rejected — error, .catch() runs

Creating a Promise

function wait(ms) {
  return new Promise((resolve) => {
    setTimeout(() => resolve('done'), ms);
  });
}

wait(1000).then(msg => console.log(msg));

Chaining API-style flow

fetch('https://api.example.com/courses')
  .then(res => {
    if (!res.ok) throw new Error('HTTP ' + res.status);
    return res.json();
  })
  .then(data => {
    console.log('Courses:', data.length);
  })
  .catch(err => {
    console.error('Failed:', err.message);
  })
  .finally(() => {
    console.log('Request finished');
  });

function validateCoupon(code) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (code === 'LEARN50') resolve({ discount: 50 });
      else reject(new Error('Invalid coupon'));
    }, 300);
  });
}

validateCoupon('LEARN50')
  .then(({ discount }) => console.log('Saved', discount + '%'))
  .catch(err => console.log(err.message));