Javascript Promises are a powerful and useful tool for handling asynchronous code in Javascript. They allow you to write code that executes in a certain order, even if some parts of the code take longer to execute than others. This is especially useful when working with APIs, databases, and other external resources that may have a delay in responding.

To understand Promises, it's important to first understand the concept of asynchronous code. In synchronous code, statements are executed one after the other, in the order that they are written. This means that the code will wait for a long-running task to complete before moving on to the next statement.

Asynchronous code, on the other hand, allows certain tasks to run in the background while the rest of the code continues to execute. This is useful when you don't want to block the main thread of execution, but it can also make it more difficult to manage the order in which certain tasks are completed.

Promises are a way to handle asynchronous code in a more manageable way. A Promise represents the result of an asynchronous operation, and can be in one of three states: pending, fulfilled, or rejected.

A Promise starts in the pending state, and then either resolves to the fulfilled state if the asynchronous operation was successful, or rejects with an error if the operation failed. This allows you to write code that handles the success or failure of an async operation in a predictable way.

Here's a simple example of how you might use a Promise:

const fetchData = () => {
  return new Promise((resolve, reject) => {
    // make an async API call
    apiCall((err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

fetchData()
  .then((data) => {
    // do something with the data
  })
  .catch((err) => {
    // handle the error
  });

In this example, we have a function called fetchData that makes an async API call using the apiCall function. The apiCall function takes a callback that will be called with an error or the data from the API.

Inside the fetchData function, we create a new Promise and pass it a function with two arguments, resolve and reject. If the apiCall function passes an error to the callback, we call the reject function with the error. If it passes data, we call the resolve function with the data.

Then, outside the fetchData function, we call the then method on the returned Promise. The then method takes a callback that will be called with the resolved value of the Promise (in this case, the data from the API). If the Promise is rejected, the catch method is called with the error.

One of the benefits of Promises is that they can be chained together. This allows you to write code that executes in a certain order, even if some of the async operations take longer to complete than others.

For example:

fetchData()
  .then((data) => {
    // do something with the data
    return processData(data);
  })
  .then((processedData) => {
    // do something with the processed data
    return storeData(processedData);
  })

In this example, we first make a call to the fetchData function, which returns a Promise. We chain a call to the then method, which takes a callback that receives the data from the fetchData function. Inside this callback, we return the result of calling the processData function with the data.

This returned value is itself a Promise, which we can chain another call to the then method on. This then method takes a callback that receives the processed data from the processData function. Inside this callback, we return the result of calling the storeData function with the processed data.

Finally, we chain another call to the then method, which takes a callback that is called when the storeData Promise is fulfilled. This allows us to write code that executes in a certain order, even if some of the async operations take longer to complete than others.

In addition to the then and catch methods, Promises also have a finally method, which is called regardless of whether the Promise is fulfilled or rejected. This can be useful for cleaning up resources or performing other tasks that need to happen regardless of the outcome of the Promise.

Promises are a powerful and useful tool for handling asynchronous code in Javascript, and are widely used in modern Javascript applications. I hope this beginner's guide to Promises has been helpful in understanding how they work and how you can use them in your own projects.