Implementing Async/Await Polyfill Using Generators in JavaScript
Tags: javascript, webdev, interview, node
Published: 2023-05-23 11:15:01

In JavaScript, `Async/Await` and `Generators` are two powerful concepts that allow us to handle asynchronous code more efficiently and intuitively. However, if for some reason you're working in an environment that doesn't support `Async/Await` syntax, you might be in need of a polyfill.

A `polyfill` is a piece of code that provides the technology that you expect the browser to provide natively. It's a way to fill in the gaps and provide modern functionality on older browsers that do not natively support it.

In this article, we will create a polyfill for Async/Await using generators in JavaScript. The idea is to create a helper function that takes a generator and recursively handles the yielded promises until the generator completes.


## Understanding Generators

Before we jump into the implementation, let's quickly review what a generator is. In JavaScript, a generator is a function that can stop midway and then continue from where it left off. When executed, it returns a special type of iterator, known as a generator object, on which we can call next to resume execution.


## The Async/Await Polyfill Implementation

Now let's see how we can implement a polyfill for Async/Await using generators. We're going to create a function called asyncGenerator, which takes a generator function as an argument and returns a new function. This new function, when called, returns a promise that resolves when the generator function completes its execution.

Here is the implementation:

```
function asyncGenerator(fn) {
  return function () {
    const _genFn = fn.apply(this, arguments);
    return new Promise((resolve, reject) => {
      function handleIterator(iterator) {
        const { value, done } = iterator;
        if (done) {
          resolve(value);
          return;
        } else {
          Promise.resolve(value).then(
            (val) => handleIterator(_genFn.next(val)),
            (error) => handleIterator(_genFn.throw(error))
          );
        }
      }
      handleIterator(_genFn.next());
    });
  };
}
```

In this code, _genFn is the generator object returned by the generator function. The handleIterator function is responsible for handling the yielded promises. If the generator is done, it resolves the promise with the yielded value. Otherwise, it resolves the yielded promise and calls handleIterator recursively with the result of the next method of the generator object.


## Testing the Polyfill

To test this implementation, we can create a simple generator function that yields a promise. This promise simulates an asynchronous operation, such as fetching data from a server.

Here is an example test:

```
function getData() {
  return new Promise((resolve) => setTimeout(() => resolve(10), 3000));
}

const asyncGenData = asyncGenerator(function* () {
  try {
    const result = yield getData();
    console.log(result,'generator data');
  } catch (error) {
    console.log(error);
  }
});

asyncGenData();
```

In this code, getData is a function that returns a promise that resolves with a value of 10 after a delay of 3000 milliseconds. asyncGenData is the generator function that yields the promise returned by getData. When we call asyncGenData, it logs the result of the promise to the console.

In conclusion, this technique of implementing `Async/Await polyfill` using generators in JavaScript can be a powerful tool for dealing with asynchronous operations, especially when dealing with environments that do not natively support Async/Await syntax. It allows us to write asynchronous code in a more synchronous and intuitive manner