# Week 3

Last week you learned about callbacks - functions that run later. You saw how nesting callbacks gets messy ("callback hell"). This week, you'll learn Promises and async/await - cleaner ways to handle asynchronous operations.

This week you'll work with a real API: the Tea Shop backend. You'll fetch data, handle errors, and chain operations - just like you would when building real backend services.

## Contents

* [Preparation](/course-content/backend/advanced-javascript/week3/preparation.md)
* [Slides](https://github.com/HackYourFuture-CPH/program/blob/main/courses/backend/advanced-javascript/week3/session-materials/slides/index.html)
* [Session Plan](/course-content/backend/advanced-javascript/week3/session-plan.md) (for mentors)
* [Session Materials](https://github.com/HackYourFuture-CPH/program/blob/main/courses/backend/advanced-javascript/week3/session-materials/README.md)
* [Assignment](/course-content/backend/advanced-javascript/week3/assignment.md)

## Learning Goals

By the end of this session, you will be able to:

* [ ] Understand why Promises exist (solving callback hell)
* [ ] Consume Promises with `.then()` and `.catch()`
* [ ] Chain multiple `.then()` calls
* [ ] Create your own Promises with `new Promise()`
* [ ] Use `async`/`await` syntax for cleaner code
* [ ] Handle errors with `try`/`catch` in async functions
* [ ] Run multiple Promises in parallel with `Promise.all()`
* [ ] Fetch data from APIs using `fetch()`

### After this session you will understand code like this

```js
// From callback hell...
validateOrder(order, (err, valid) => {
  if (err) return handleError(err);
  calculateTotal(order, (err, total) => {
    if (err) return handleError(err);
    checkStock(order, (err, inStock) => {
      if (err) return handleError(err);
      // deeply nested...
    });
  });
});

// ...to clean async/await
async function processOrder(order) {
  const valid = await validateOrder(order);
  const total = await calculateTotal(order);
  const inStock = await checkStock(order);
  return { valid, total, inStock };
}
```


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