then()

ES6+

Attaches callbacks for the resolution and/or rejection of the Promise.

Syntax

promise.then(onFulfilled, onRejected)

Parameters

onFulfilled Function

A function called when the Promise is fulfilled

onRejected Function optional

A function called when the Promise is rejected

Return Value

Promise

A new Promise that resolves to the return value of the callback

Examples

JavaScript

const promise = Promise.resolve(42);
promise.then(value => {
  console.log(value);
});

Output:

// 42

📌 When to Use

Use then() to handle successful async operations, transform data through chaining, or sequence multiple async operations.

⚠️ Common Mistakes

Forgetting to return values in then() chains, breaking the chain with undefined.

Not handling rejections - unhandled promise rejections can crash Node.js applications.

Creating unnecessary nesting instead of chaining - leads to callback hell.

✅ Best Practices

Use async/await for cleaner code, but understand then() for legacy codebases.

Always end chains with catch() or use the second argument of then() sparingly.

⚡ Performance Notes

Each then() creates a new Promise object. For high-performance scenarios, minimize chain length. Consider batching with Promise.all() for parallel operations.

🌍 Real World Example

API Data Transformation Pipeline

Chain API calls and transform data through multiple processing stages.

function fetchUserWithPosts(userId) {
  return fetch(`/api/users/${userId}`)
    .then(response => {
      if (!response.ok) throw new Error('User not found');
      return response.json();
    })
    .then(user => {
      return fetch(`/api/users/${userId}/posts`)
        .then(res => res.json())
        .then(posts => ({ ...user, posts }));
    })
    .then(userWithPosts => {
      // Transform data
      return {
        ...userWithPosts,
        postCount: userWithPosts.posts.length,
        recentPost: userWithPosts.posts[0] || null
      };
    });
}

fetchUserWithPosts(1).then(console.log).catch(console.error);

Related Methods

catch() finally()