🚀 Mastering JavaScript Promises for FAANG Interviews

A Complete Guide to Patterns, Internals & Polyfills (with Examples)

JavaScript Promises aren’t just another async concept — in FAANG interviews, they are a playground for testing deep technical understanding.
You’re not only expected to use Promises…
You’re expected to rebuild them.

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This guide breaks down the most important Promise patterns you must master to crack high-level JavaScript interviews.

🔹 Pattern 1: Promise Basics

Before you dive into polyfills or chaining, you must know how Promises behave.

✅ What is a Promise?

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation.

🔥 Three core states

• Pending — initial state
• Fulfilled — operation completed successfully
• Rejected — operation failed

⚡ Key Methods

• Promise.resolve(value) → returns a fulfilled Promise
• Promise.reject(error) → returns a rejected Promise

🔹 Pattern 2: Promise Chaining

Chaining is a powerful concept used heavily in async flows.

👉 How chaining works internally

Each _.then()_ returns a new Promise, making chaining possible.

👉 Why return inside .then()?

Without returning a value/Promise, the next _.then()_ receives _undefined_.

👉 How values flow through the chain

Each _.then()_ receives the resolved result of the previous one.

🔹 Pattern 3: Error Handling

Error handling is a favorite FAANG topic.

.catch() vs .then(null, errorHandler)

• .catch() is cleaner and recommended.
• .then() error handlers only catch errors from the chain above them.

What if you don’t catch an error?

Uncaught errors propagate down the chain until found — or lead to an unhandled rejection.

Can the same error be caught twice?

Yes — if you rethrow it intentionally.

🔹 Pattern 4: Advanced Promise Methods

Understanding differences between Promise helpers is crucial.

⭐ Promise.all

Waits for all Promises → fails fast.

⭐ Promise.allSettled

Waits for all Promises → never fails.

⭐ Promise.any

Resolves with first success → fails only if all fail.

⭐ Promise.race

Returns the first Promise settled (success or failure).

📌 Real-World Use Cases

• all → load multiple APIs before rendering
• race → timeout logic
• any → best-effort fallbacks
• allSettled → analytics, logging, bulk job results

🔹 Pattern 5: FAANG-Favorite Topic — Polyfills

You must be able to implement these:

• Custom _Promise.all_ polyfill
• Custom _Promise.race_ polyfill
• Custom _Promise.any_ polyfill
• Custom _Promise.allSettled_ polyfill
• Custom _Promise_ constructor
• Polyfill for _.then()_ and _.catch()_

These questions test your understanding of:

• Promise queueing
• State management
• Resolution pipelines
• Error propagation

🔹 Pattern 6: Async/Await

Async/Await is just syntactic sugar over Promises — but with different behavior.

How does it work internally?

async functions always return a Promise.
await pauses execution until the Promise resolves.

Error handling difference

• Promises → .catch()
• Async/await → try…catch

Can async/await be used with Promise.all?

Absolutely — it’s the recommended pattern for concurrent execution.

💡 Final Interview Tip

FAANG loves polyfills.
If you can recreate Promise methods from scratch, you demonstrate mastery of:

• Event loops
• Microtasks
• Asynchronous pipelines
• JavaScript internals

It’s one of the strongest signals of a top-tier engineer.

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