JavaScript: Promise
Tags: javascript, interview
Published: 2023-10-19 01:40:03

## How to use JavaScript Promises in the industry :-
**Ecommerce:**

> Promises are a way to handle asynchronous operations more cleanly and manage errors effectively.

- E-commerce platforms often rely on external APIs for various functionalities like product catalog, payment processing, or shipping calculations. When making API requests, use the fetch API or a library like Axios that returns promises.


```
fetch('https://api.example.com/products')
  .then(response => response.json())
  .then(data => {
    // Handle the API response data
  })
  .catch(error => {
    // Handle errors
  });
```

- Promises are a fundamental concept in asynchronous programming in JavaScript.

- They provide a way to handle asynchronous operations more cleanly and manage the flow of asynchronous code. 


****The core concepts of promises include:

1. **States**:
   - Promises have three possible states:
     - **Pending**: The initial state when a promise is created, representing that the operation is ongoing or hasn't completed yet.
     - **Fulfilled**: The state when the asynchronous operation is successfully completed, and a result value is available.
     - **Rejected**: The state when the asynchronous operation encounters an error or fails, and an error reason is provided.

2. **Callbacks**:
   - Promises use two callback functions to handle their states:
     - **resolve()**: A function called when the promise is successfully fulfilled. It passes a result value to any attached `.then()` handlers.
     - **reject()**: A function called when the promise is rejected due to an error. It passes an error reason to any attached `.catch()` handlers.

3. **Chaining**:
   - Promises allow you to chain multiple asynchronous operations together using `.then()`. This enables you to specify a sequence of tasks to execute when the promise is fulfilled.

   ```javascript
   someAsyncFunction()
     .then(result => {
       // Handle the result
       return anotherAsyncFunction(result);
     })
     .then(finalResult => {
       // Handle the final result
     })
     .catch(error => {
       // Handle errors from any of the steps
     });
   ```

4. **Error Handling**:
   - Promises provide a centralized way to handle errors using `.catch()`. Errors thrown in any `.then()` block will propagate down to the nearest `.catch()` handler.

   ```javascript
   someAsyncFunction()
     .then(result => {
       // Handle the result
       throw new Error('An error occurred');
     })
     .catch(error => {
       // Handle the error
     });
   ```

5. **Promise.all()**:
   - You can use `Promise.all()` to wait for multiple promises to fulfill in parallel. It resolves when all the provided promises have resolved, providing an array of their results.

   ```javascript
   const promises = [promise1, promise2, promise3];
   Promise.all(promises)
     .then(results => {
       // Handle the results
     })
     .catch(error => {
       // Handle errors from any of the promises
     });
   ```

6. **Async/Await** (ES6 and later):
   - ES6 introduced the `async/await` syntax, which simplifies working with promises, making asynchronous code look more like synchronous code.

   ```javascript
   async function fetchData() {
     try {
       const result = await someAsyncFunction();
       // Handle the result
     } catch (error) {
       // Handle errors
     }
   }
   ```

Promises are a crucial part of modern JavaScript and are widely used in web development for handling asynchronous operations such as AJAX requests, file I/O, and more. They promote cleaner, more maintainable code by providing a structured way to deal with asynchronous behavior and errors.