Promises vs Async/Await: Interview Deep Dive

Promises in 30 Seconds

A Promise represents a value that may be available now, later, or never. States: pending, fulfilled, rejected.

const fetchUser = () =>
  fetch("/api/user").then((res) => {
    if (!res.ok) throw new Error("Failed");
    return res.json();
  });

Async/Await

Syntactic sugar over Promises — same behavior, cleaner code.

async function loadUser() {
  try {
    const res = await fetch("/api/user");
    if (!res.ok) throw new Error("Failed");
    return await res.json();
  } catch (error) {
    console.error(error);
    throw error;
  }
}

Top Interview Questions

Q: What does Promise.all do if one rejects?

It immediately rejects with the first rejection. Use Promise.allSettled when you need all results.

Q: Implement a simple Promise

class MyPromise {
  constructor(executor) {
    this.state = "pending";
    this.value = undefined;
    this.handlers = [];

    const resolve = (value) => {
      if (this.state !== "pending") return;
      this.state = "fulfilled";
      this.value = value;
      this.handlers.forEach((h) => h.onFulfilled(value));
    };

    executor(resolve);
  }

  then(onFulfilled) {
    if (this.state === "fulfilled") onFulfilled(this.value);
    else this.handlers.push({ onFulfilled });
  }
}

Q: Sequential vs parallel async?

// Parallel — faster
const [a, b] = await Promise.all([fetchA(), fetchB()]);

// Sequential — when B depends on A
const a = await fetchA();
const b = await fetchB(a.id);