Javascript promise mechanism, how it work?
Published: 2024-02-07 14:47:05

JavaScript promises are objects that represent the eventual completion or failure of an asynchronous operation and its resulting value. Behind the scenes, promises are implemented using a combination of the event loop, microtask queue, and callbacks. Here's how the promise mechanism works:

1. **Creation of Promise**: When you create a new promise, the executor function you provide is executed immediately. This executor function takes two parameters: `resolve` and `reject`. You use these functions to indicate the successful completion or failure of the asynchronous operation.

   ```javascript
   const promise = new Promise((resolve, reject) => {
       // Asynchronous operation
       if (/* operation is successful */) {
           resolve(result); // Resolve the promise with the result
       } else {
           reject(error); // Reject the promise with an error
       }
   });
   ```

2. **Pending Promise State**: Initially, the promise is in a pending state. This means that the asynchronous operation has not yet completed, and the promise is neither fulfilled nor rejected.

3. **Execution of Asynchronous Operation**: The asynchronous operation inside the promise executor is performed. This could be fetching data from a server, reading a file, or any other asynchronous task.

4. **Transition to Fulfilled or Rejected State**: Once the asynchronous operation completes, the promise transitions to either the fulfilled or rejected state, depending on whether `resolve` or `reject` was called inside the executor function.

   - If `resolve(result)` is called, the promise transitions to the fulfilled state, and the `then` method callbacks (registered with `.then()`) associated with the promise are executed, receiving the resolved value (`result`) as an argument.
   - If `reject(error)` is called, the promise transitions to the rejected state, and the `catch` method callbacks (registered with `.catch()`) or the `onRejected` callback of `.then()` associated with the promise are executed, receiving the rejection reason (`error`) as an argument.

5. **Microtask Queue and Event Loop**: Promise callbacks (`.then()` and `.catch()` handlers) are queued as microtasks. Microtasks have a higher priority than regular tasks in the event loop, ensuring that promise callbacks are executed before any other code, such as I/O operations or UI rendering.

6. **Chaining Promises**: Promises allow for chaining using `.then()` and `.catch()` methods, which return new promises. This enables you to handle asynchronous operations sequentially or in a more complex manner.

   ```javascript
   promise.then(result => {
       // Handle success
       return nextStep(result);
   }).then(nextResult => {
       // Handle next step
   }).catch(error => {
       // Handle error
   });
   ```

7. **Error Propagation**: If an error occurs in any `.then()` handler or the executor function itself, and it's not caught by a `.catch()` handler, the error propagates down the promise chain until it's caught by a `.catch()` handler or reaches the global error handler.

By using promises, JavaScript provides a more structured and convenient way to work with asynchronous code, making it easier to manage complex asynchronous operations and handle errors effectively.