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Async JavaScript

Master asynchronous JavaScript with Promises, async/await, and error handling

8 min read
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Official Docs

Callbacks

Basic Callbacks

Traditional pattern for handling asynchronous operations with callback functions

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💡 Callbacks are functions passed as arguments to be executed later
⚠️ Can lead to "callback hell" with deep nesting
📌 Error-first convention: first parameter is for errors
🔄 Still used in many Node.js APIs and older libraries

Callback Hell

The problem of deeply nested callbacks and how to avoid it with modern patterns

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⚠️ Deeply nested callbacks create pyramid of doom
💡 Each level depends on the previous, making it hard to read
✅ Solved by promises and async/await patterns
🔄 Named functions can help flatten callback structures

Timers & Event Loop

Timer Functions

Schedule code execution after delays or at regular intervals using built-in timer functions

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💡 setTimeout executes once after a delay, setInterval repeats
⚠️ Timers are not guaranteed to execute at exact times
🔄 Always clear intervals and timeouts when no longer needed
⚡ Use setTimeout with 0ms to defer execution to next tick

Event Loop

Understand how JavaScript handles asynchronous operations through the event loop mechanism

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💡 JavaScript is single-threaded but uses an event loop for async operations
⚡ Microtasks (promises) have priority over macrotasks (setTimeout)
📌 Understanding the event loop helps debug timing issues
🔄 Call stack must be empty before event loop processes callbacks

Promises

Creating Promises

Create a new Promise that resolves or rejects based on asynchronous operations

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💡 A Promise is an object representing a future value that may not be available yet
⚡ Promises have three states: pending, fulfilled (resolved), or rejected
✅ Better than callbacks for handling async operations and avoiding callback hell
🔄 Once settled (fulfilled or rejected), a promise state cannot change

Promise Combinators

Utility methods for coordinating multiple promises with different execution strategies

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💡 Promise.all fails fast - rejects if any promise rejects
✅ Promise.allSettled waits for all regardless of outcome
⚡ Promise.race returns first to settle (resolve or reject)
🔄 Promise.any returns first to resolve, ignores rejections

Async/Await

Basic Async/Await

Modern syntax for writing asynchronous code that looks and behaves like synchronous code

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💡 Async/await is syntactic sugar built on top of promises
⚡ Makes asynchronous code look and behave like synchronous code
⚠️ Can only use await inside async functions
✅ Easier to read and debug than promise chains

Advanced Async Patterns

Complex patterns for handling concurrency, throttling, and async flow control

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💡 Throttling limits function calls to a maximum rate
⚡ Debouncing delays execution until after calls stop
✅ Queues help manage concurrent operation limits
🔄 Async iterators enable processing of streaming data

Abort & Error Handling

AbortController

Cancel ongoing asynchronous operations like fetch requests or long-running tasks

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💡 Allows you to cancel fetch requests and other async operations
⚡ Prevents unnecessary work when user navigates away
✅ Essential for cleaning up long-running operations
🔄 Can be reused by creating new controller instances

Error Handling Strategies

Best practices and patterns for handling errors in asynchronous JavaScript code

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💡 Always handle promise rejections to avoid unhandled errors
⚠️ Async errors don't bubble up like synchronous errors
✅ Use try-catch with async/await for cleaner error handling
🔄 Consider implementing retry logic for transient failures

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