Polyfill for Async Await in JavaScript - Step by Step Explanation

Async Await in JavaScript provides a simple way to handle async operations synchronously. This removes the issues that come with callback hell while using promises to handle async operations in JavaScript. This is one of the most commonly asked questions to the senior devs to evaluate their in-depth understanding.

Understanding a polyfill for Async Await (the common approach is generator + Promise) is a great way to show deep knowledge of the event loop, Promise resolution, error propagation, and how transpilers like Babel work. This post walks you through a clear polyfill, examples, test cases, and a step-by-step explanation.

Table of contents

• Polyfill for Async Await in JavaScript
  • Understand the async await in JavaScript
  • Expected async await behaviour - Test Cases
• async await polyfill code in JavaScript(ES6) with example
  • Step-by-Step async await Polyfill Explanation
• Final Notes
• Learn Next

Polyfill for Async Await in JavaScript

Polyfill of Async Await will help to understand and reproduces the major runtime behaviours of Async Await for example returning a Promise, awaiting Promises (or non-Promise values), handling throw/try-catch, and preserving sequential control flow by pausing execution until an awaited Promise settles. This way the async await provides a sequential execution of the async code.

Understand async await in JavaScript

Let's first understand how async await works in the sequential and parallel calls. This will help to create the required test cases which async await polyfill needs to satisfy.

Example:

const delay = (ms, value, fail = false) =>
  new Promise((resolve, reject) => {
    setTimeout(() => (fail ? reject(value) : resolve(value)), ms);
  });

// Example 1: Sequential awaits (runs one after the other)
async function sequentialFetch() {
  const a = await delay(300, 1);
  const b = await delay(300, 2);
  return a + b; // ~600ms
}

// Example 2: Parallel with Promise.all
async function parallelFetch() {
  const [a, b] = await Promise.all([delay(300, 1), delay(300, 2)]);
  return a + b; // ~300ms
}

// Example 3: Error handling with await
async function fetchWithCatch() {
  try {
    const v = await delay(100, 'Error', true); // will reject
    return v;
  } catch (err) {
    return 'caught: ' + err;
  }
}

// Usage
sequentialFetch().then(console.log); // 3
parallelFetch().then(console.log);   // 3
fetchWithCatch().then(console.log);  // caught: Error

Expected async await behaviour (Test Cases)

Its important to know what all to expect from async await polyfill, and make sure it satisfies these behaviours (or we can say test cases):

1. async returns a Promise: Calling the wrapped function should immediately return a Promise.
2. Await on non-Promise: await 42 should give 42 (wrapped by Promise.resolve).
3. Sequential execution: const a = await p1; const b = await p2; should wait for p1 to settle before starting p2's effective continuation.
4. Parallel via Promise.all: await Promise.all([p1,p2]) should run both p1 and p2 concurrently and wait for both.
5. Error propagation: If an awaited Promise rejects, the returned Promise should reject (unless caught with try/catch).
6. Try/catch: A try around await should catch rejections from the awaited Promise.
7. Return value: Returning a value from the async function resolves the returned Promise with that value.
8. Thrown exceptions: Throwing inside the async function rejects the returned Promise.

async await polyfill code in JavaScript(ES6) with example

Below is a widely used pattern (conceptually equivalent to Babel's _asyncToGenerator) that implements async / await using generator functions + Promises. It's intentionally small so you can explain it in an interview.

/**
 * Converts a generator function into an async-like function.
 * @param {Function} genFn - The generator function.
 * @returns {Function} A function that returns a Promise.
 */
export function asyncToGenerator(genFn) {
  return function (...args) {
    const self = this; // preserve `this` context
    const gen = genFn.apply(self, args); // initialize generator

    return new Promise((resolve, reject) => {
      // Recursive step function
      function step(nextFn, arg) {
        let result;
        try {
          // Advance the generator
          result = gen[nextFn](arg);
        } catch (err) {
          // If generator throws, reject outer promise
          reject(err);
          return;
        }

        const { value, done } = result;

        if (done) {
          // Generator completed
          resolve(value);
        } else {
          // Await value (handle normal values too)
          Promise.resolve(value).then(
            val => step("next", val),
            err => step("throw", err)
          );
        }
      }

      // Start execution
      step("next");
    });
  };
}

1. Usage Example - Simulated API calls

• Fetch a user
• Fetch that user’s posts
• Combine the result and show it on the UI

/** This is what we generally call the API
async function getUserPosts() {
  const user = await fetchUser();
  const posts = await fetchPosts(user.id);
  return { ...user, posts };
}
*/

// Here is how we can call using the async Polyfill.
const fetchUser = () =>
  new Promise(res => setTimeout(() => res({ id: 1, name: "Anuj Sharma" }), 500));

const fetchPosts = (userId) =>
  new Promise(res =>
    setTimeout(
      () =>
        res([
          { id: 101, userId, title: "FrontendGeek" },
          { id: 102, userId, title: "Building Polyfills" },
        ]),
      500
    )
  );

// Using our polyfill
const getUserPosts = asyncToGenerator(function* () {
  const user = yield fetchUser(); // wait for user
  const posts = yield fetchPosts(user.id); // wait for posts
  return { ...user, posts }; // final return value
});

// Execute like an async function
getUserPosts().then(console.log).catch(console.error);

// Output after 1 sec
{
  id: 1,
  name: "Anuj Sharma",
  posts: [
    { id: 101, userId: 1, title: "FrontendGeek" },
    { id: 102, userId: 1, title: "Building Polyfills" }
  ]
}

2. Usage Example - Parallel API call using Promise.all()

You can also yield a Promise.all() for parallel API calls, exactly like native async/await:

const getUserAndSettings = asyncToGenerator(function* () {
  const [user, settings] = yield Promise.all([
    fetchUser(),
    new Promise(res => setTimeout(() => res({ theme: "dark" }), 500))
  ]);
  return { ...user, settings };
});

getUserAndSettings().then(console.log);

Step-by-Step async await Polyfill Explanation

Below is a detailed breakdown of every step inside our asyncToGenerator helper and why it exists.

Step 1: Define the Polyfill Function

We start by creating a function (e.g., asyncPolyfill) that takes a generator function as input. This generator will represent our “async” function that yields promises.

Step 2: Initialize the Generator

Inside the polyfill, we call the generator function to get a generator object. This gives us access to .next() and .throw() methods to control the flow.

Step 3: Create a Recursive step() Function

We define a helper function step() to move through the generator sequence. This function will:

• Call next() to get the next value.
• Check if the generator is done.
• If not done, handle the yielded promise.

Step 4: Handle Promises Returned by Yield

If the generator yields a promise, we wait for it to resolve. Once it resolves, we feed the resolved value back into the generator via next(value).

If it rejects, we handle the error using throw(error) to keep the same behaviour as native async/await.

Step 5: Continue Until Completion

We recursively call step() until the generator signals it’s done (done: true). At that point, we resolve the final promise with the generator’s return value.

Step 6: Return a Promise

Finally, our polyfill returns a Promise so that the entire async function behaves like a native one, allowing us to use .then() or await it externally.

Final notes

This polyfill is intentionally small for clarity and interview explanation. Real transpilers (Babel + regenerator) add more features but the core idea is the same: generators + Promise-driven stepping produce the async await polyfill in JavaScript.

Learn Next

1. Promise polyfill in JavaScript
2. Promise.all polyfill in JavaScript
3. Promise.race polyfill in JavaScript
4. Best Polyfill resources to ace JavaScript Interview