Introduction#
JavaScript is single-threaded by design, yet it powers highly interactive, non-blocking applications. This apparent contradiction is resolved through asynchronous programming. Understanding how JavaScript handles async operations is essential for writing scalable, maintainable, and performant code.
This article walks step by step through:
What asynchronous JavaScript really means
The Event Loop and the Runtime Model
Callback-based patterns and their limitations
Promises as a structured alternative
async/awaitfor clean, modern async code
The goal is not only to explain how these work, but why each evolution exists.
What Is Asynchronous JavaScript?#
In synchronous code, each operation blocks the next until it completes:
A task starts
JavaScript waits for it to finish
Only then does the next task run
Asynchronous code allows JavaScript to:
Start a long-running task (network request, timer, file read)
Continue executing other code
Handle the result later, when it is ready
This is critical for:
Network requests (APIs)
Timers and delays
User interactions
Animations
JavaScript achieves this using the event loop, callbacks, and microtask queues—but developers interact with it through higher-level abstractions.
Why Asynchronous JavaScript Matters#
Fetching data from APIs can take significant time. Async execution keeps the UI responsive.
Asynchronous programming allows JavaScript to initiate these operations without blocking the main thread. While the operation runs in the background, JavaScript continues executing other tasks, keeping applications responsive and interactive.
This is especially important for:
Network requests
Fetching data from APIs can take hundreds of milliseconds or even seconds depending on network conditions. Asynchronous execution allows the application to remain responsive while waiting for the response.
User interface responsiveness
Browsers rely on the main thread for rendering and interaction. Blocking this thread with long operations would freeze the UI, preventing scrolling, clicks, or animations.
Timers and scheduled tasks
Functions like setTimeout and setInterval rely on asynchronous behavior to schedule code execution without interrupting the
rest of the program.
Concurrent operations
Multiple asynchronous tasks—such as loading images, fetching data, and handling user input—can progress independently without forcing sequential execution.
In short, asynchronous programming enables JavaScript to manage waiting efficiently. Instead of stopping the entire program while a task completes, JavaScript schedules the result to be handled later, ensuring applications remain smooth and responsive.
The JavaScript Runtime Model: How Asynchronous Code Actually Runs#
To truly understand asynchronous JavaScript, it helps to understand how JavaScript executes code under the hood.
Single-Threaded Nature#
JavaScript runs on a single main thread, meaning only one piece of JavaScript executes at a time. Long synchronous tasks would block the UI, which is why asynchronous APIs exist.
Web APIs#
Operations like fetch or setTimeout are offloaded to Web APIs provided by the browser. These APIs run outside the main thread. Once completed, they send their results back to the queues.
The Event Loop#
The event loop continuously checks:
Is the call stack empty?
Are there pending tasks in the queues?
If yes, it moves queued callbacks onto the call stack.
There are two important queues:
Macro task queue (timers, events)
Microtask queue (promises,
queueMicrotask)
Microtasks always run before the next macro task.
The Golden Rule: The Event Loop will empty the entire Microtask Queue before moving to the next Macrotask.
Below is the JavaScript event loop diagram showing call stack, web APIs, task queue, and microtask queue
Call Stack
↓
Web APIs (fetch, setTimeout)
↓
Task Queue
↓
Microtask Queue (priority)
↓
Event Loop
Event Loop Execution Flow (Visual Timeline)#
Understanding the Event Loop becomes much easier when visualized as a step-by-step execution flow.
┌──────────────────────┐
│ Call Stack │
│ (Executes JS code) │
└─────────┬────────────┘
│
▼
┌──────────────────────┐
│ Web APIs │
│ (Timers, fetch, DOM) │
└─────────┬────────────┘
│
▼
┌──────────────────────────────┐
│ Task Queues │
│ ┌────────────────────────┐ │
│ │ Microtask Queue │ │ ← Promises, queueMicrotask
│ └────────────────────────┘ │
│ ┌────────────────────────┐ │
│ │ Macrotask Queue │ │ ← setTimeout, events
│ └────────────────────────┘ │
└─────────┬────────────────────┘
│
▼
┌──────────────────────┐
│ Event Loop │
│ Moves tasks to stack │
└──────────────────────┘
How Execution Happens#
JavaScript executes code from the Call Stack
Async operations are offloaded to Web APIs
-
When complete, callbacks are queued:
Promises → Microtask Queue
Timers/events → Macrotask Queue
-
The Event Loop checks:
If the call stack is empty
Then executes all microtasks first
Then executes one macrotask
The cycle repeats continuously
Key Insight#
The Event Loop always prioritizes the Microtask Queue over the Macrotask Queue.
This is why promise callbacks run before setTimeout, even if the timeout is 0.
The Evolution of JavaScript Async Patterns#
As JavaScript applications grew in complexity, the language evolved to provide better ways of managing asynchronous operations. Each generation of async patterns solved problems introduced by the previous one.
1. Callbacks#
Callbacks were the original solution for asynchronous operations in JavaScript. A callback is simply a function passed into another function that executes once a task finishes.
While callbacks work well for simple cases, complex workflows quickly lead to deeply nested code structures, often referred to as callback hell.
2. Promises#
Promises were introduced to provide a more structured way to represent asynchronous results. Instead of passing functions around manually, a promise represents a value that will eventually be available.
Promises allow developers to chain operations using .then() and handle failures using .catch(), significantly improving readability
and error management.
3. Async/Await#
async and await, introduced in ES2017, build on top of promises to make asynchronous code look synchronous. Instead of chaining
.then() calls, developers can write sequential-looking code while still executing asynchronously.
This approach improves readability, simplifies error handling with try...catch, and aligns asynchronous code with familiar control
flow patterns.
Summary of the Evolution#
The progression of asynchronous JavaScript can be summarized as:
Callbacks → Promises → Async/Await
Each step reduces complexity, improves readability, and makes asynchronous logic easier to reason about in large applications.
Callbacks: The Original Pattern#
A callback is a function passed as an argument to another function and executed after an asynchronous operation completes. Callbacks were the earliest and most common way to handle async behavior in JavaScript.
Basic Callback Example#
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function fetchData(callback) {
setTimeout(() => {
callback("Data loaded");
}, 1000);
}
fetchData((result) => {
console.log(result);
});
What’s happening here:
setTimeoutsimulates an asynchronous operationThe callback is invoked once the operation finishes
Execution continues without blocking the main thread
For small, simple async tasks, callbacks are straightforward and effective.
Common Callback Patterns#
Callbacks remain widely used today, especially in:
Event listeners
Streaming and data flow APIs
Low-level or performance-critical libraries
Error-First Callbacks#
Many APIs (notably Node.js) use the error-first callback convention, where the first argument represents a potential error.
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readFile(path, (err, data) => {
if (err) {
console.error(err);
return;
}
console.log(data);
});
This pattern ensures errors are explicitly handled before accessing results.
Limitations of Callbacks#
As application complexity increases, callbacks begin to show serious drawbacks:
No built-in chaining mechanism
Manual error propagation required
Inversion of control (you hand over execution flow)
Readability issues when nesting callbacks deeply
These limitations led to callback hell, motivating the evolution toward Promises and async/await.
Callback Hell: When Things Go Wrong#
Problems arise when multiple async operations depend on each other.
Nested Callbacks Example#
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getUser(id, (user) => {
getOrders(user.id, (orders) => {
getOrderDetails(orders[0], (details) => {
console.log(details);
});
});
});
This pattern is known as callback hell.
Why Callback Hell Is a Problem#
Code becomes deeply nested and hard to read
Error handling is repetitive and fragile
Logic flow is difficult to follow
Refactoring becomes risky
Callback hell is not just about indentation—it is about loss of clarity.
Promises: A Better Abstraction#
A Promise represents the eventual result of an asynchronous operation. It may be pending, fulfilled, or rejected. It provides a structured way to manage asynchronous operations and replace deeply nested callbacks.
Promise States:
A Promise is always in exactly one of these states:
Pending – initial state.
Fulfilled (Resolved) – The operation completed successfully.
Rejected – The operation failed.
Once settled (fulfilled or rejected), the state is immutable.
Promises allow you to chain operations using .then() and handle errors gracefully with .catch().
Creating and Consuming Promises#
Creating a Promise
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const fetchData = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Data loaded");
}, 1000);
});
};
Consuming a Promise
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fetchData()
.then((result) => {
console.log(result);
})
.catch((error) => {
console.error(error);
}).finally(() => {
console.log("Cleanup logic here")
});
Promises flatten nested callbacks and centralize error handling, significantly improving readability.
Promise Chaining#
Promises can be chained to represent sequential asynchronous steps.
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getUser(id)
.then((user) => getOrders(user.id))
.then((orders) => getOrderDetails(orders[0]))
.then((details) => console.log(details))
.catch((error) => console.error(error));
Benefits of Promise Chaining#
Linear, readable flow
Single error-handling path
Easier composition of async logic
⚠️ However, long chains can still become verbose — a limitation later addressed by async/await.
Promise Internals and Behavior#
Understanding Promise behavior is critical for avoiding subtle bugs.
Promises Are Eager
Execution begins immediately when a Promise is created
Promises represent a result, not the operation itself
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Promise.resolve(Promise.resolve(42)).then(console.log);
A common “gotcha” for learners: while the .then() block is asynchronous, the code inside the Promise constructor runs synchronously and immediately.
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console.log("1. Before Promise");
new Promise((resolve) => {
console.log("2. Inside Promise Constructor (Synchronous!)");
resolve();
});
console.log("3. After Promise");
Promise Resolution Rules#
Resolving with a value fulfills the Promise
Resolving with another Promise adopts its state
Throwing an error automatically rejects the Promise
These rules make Promise composition predictable and consistent.
Promise Utility Methods#
JavaScript provides static Promise methods for common async coordination patterns.
Promise.all#
Promise.all is fail-fast. It takes an array of promises and waits for all of them to succeed. However, if any single promise
rejects, the entire operation fails immediately.
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// Captures an array of results; fails if any task rejects
const results = await Promise.all([taskA(), taskB()]);
// Pro-Tip: Destructure for immediate access
const [dataA, dataB] = await Promise.all([taskA(), taskB()]);
Best Use Case: When the tasks are dependent on each other. For example, you need both a user_id and an access_token to load a dashboard. If you don’t get both, the dashboard is useless.
Promise.allSettled#
Promise.allSettled waits for every promise to finish, regardless of whether they succeeded or failed. It returns
an array of objects describing the outcome of each.
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// Captures an array of outcome objects {status, value/reason}
const outcomes = await Promise.allSettled([taskA(), taskB()]);
Best Use Case: When the tasks are independent. For example, loading five different widgets on a page. If the “Weather” widget fails, you still want the “Stock Market” and “News” widgets to display.
Promise.race#
The Promise settles (either resolves or rejects) as soon as the first promise in the group settles.
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// Settles as soon as the first promise settles (winner or error)
const firstSettled = await Promise.race([taskA(), taskB()]);
Best Use Case: Implementing a timeout for a network request. If the request doesn’t finish in 5 seconds, the “timeout” promise wins the race and rejects.
Promise.any#
The Promise resolves as soon as the first promise fulfills (succeeds). It ignores rejections unless every promise in the group fails.
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// Resolves as soon as the first promise fulfills (ignores errors)
const firstSuccess = await Promise.any([taskA(), taskB()]);
Best Use Case: Requesting data from three different “mirror” servers. You don’t care if two of them are down; you only need the first successful response.
The queueMicrotask() API#
Sometimes you need to schedule a function to run asynchronously but immediately after the current task, without the
overhead of creating a full Promise. This is where queueMicrotask() shines.
Example Comparison:
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console.log("Start");
setTimeout(() => console.log("Macrotask (Timeout)"), 0);
queueMicrotask(() => console.log("Microtask (Direct)"));
Promise.resolve().then(() => console.log("Microtask (Promise)"));
console.log("End");
// Result: Start -> End -> Microtask (Direct) -> Microtask (Promise) -> Macrotask (Timeout)
Microtask vs Macrotask: Execution Timeline#
To truly understand execution order, consider this example:
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console.log("1. Start");
setTimeout(() => {
console.log("2. setTimeout (Macrotask)");
}, 0);
Promise.resolve().then(() => {
console.log("3. Promise (Microtask)");
});
queueMicrotask(() => {
console.log("4. queueMicrotask (Microtask)");
});
console.log("5. End");
Step-by-Step Execution#
Step 1: Run synchronous code
→ "1. Start"
→ "5. End"
Step 2: Process Microtasks (ALL of them)
→ "3. Promise (Microtask)"
→ "4. queueMicrotask (Microtask)"
Step 3: Process ONE Macrotask
→ "2. setTimeout (Macrotask)"
Final Output#
1. Start
5. End
3. Promise (Microtask)
4. queueMicrotask (Microtask)
2. setTimeout (Macrotask)
Visual Timeline#
Call Stack Execution:
[ Start → End ]
Microtask Queue:
[ Promise → queueMicrotask ]
Macrotask Queue:
[ setTimeout ]
Execution Order:
Stack → Microtasks → Macrotask
Important Takeaways#
Microtasks always run before macrotasks
All microtasks are executed completely before moving on
Even
setTimeout(fn, 0)runs after microtasksqueueMicrotaskand Promises behave similarly in priority
Microtask Starvation Warning#
Because microtasks have higher priority, continuously adding microtasks can block macrotasks and even UI rendering.
Example:
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function loop() {
queueMicrotask(loop);
}
loop();
This creates an infinite microtask loop, preventing the event loop from ever reaching macrotasks.
Async/Await: Modern Asynchronous JavaScript#
Introduced in ES2017, async and await are “syntactic sugar” built on top of Promises. They allow you to write asynchronous code
that looks and behaves like synchronous code, making it significantly more readable.
async: Declares that a function returns a promise.await: Pauses the execution of the function until the promise resolves.
Basic Example#
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async function fetchData() {
const result = await new Promise((resolve) => {
setTimeout(() => resolve("Data loaded"), 1000);
});
console.log(result);
}
Key points:
asyncfunctions always return a promiseawaitpauses execution inside the function until the promise resolvesThe main thread is not blocked
Refactoring Promise Chains with Async/Await#
Promise Version#
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getUser(id)
.then((user) => getOrders(user.id))
.then((orders) => getOrderDetails(orders[0]))
.then((details) => console.log(details));
Async/Await Version#
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async function loadOrderDetails(id) {
const user = await getUser(id);
const orders = await getOrders(user.id);
const details = await getOrderDetails(orders[0]);
console.log(details);
}
Why Async/Await Is Preferred#
Reads top-to-bottom
Easier debugging
Familiar control flow (try/catch, loops)
Error Handling with Async/Await#
Error handling becomes straightforward using try...catch.
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async function loadData() {
try {
const data = await fetchData();
console.log(data);
} catch (error) {
console.error(error);
}
}
This mirrors synchronous error handling, making code more predictable.
Async/Await Under the Hood#
async / await does not block execution.
Internally:
awaitsplits the function into promise-based stepsExecution resumes via the microtask queue
This explains why:
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console.log('start');
await Promise.resolve();
console.log('end');
still behaves asynchronously.
Top-Level Await#
Traditionally, the await keyword could only be used inside an async function. This restriction meant developers often had to wrap
initialization code inside an async function or an asynchronous IIFE.
Modern JavaScript modules introduced Top-Level Await, allowing await to be used directly at the module level without wrapping it
inside an async function.
This feature simplifies startup logic in ES modules, especially when loading configuration data, initializing resources, or fetching initial application state.
Example#
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const data = await fetch("/api/data").then(response => response.json());
console.log(data);
Where Top-Level Await Works#
Top-level await is supported in:
ES modules (
type="module"in browsers)Modern Node.js environments
Build systems that support ES module syntax
When to Use It#
Top-level await is useful for:
Loading configuration before application startup
Fetching initialization data
Dynamic module loading
Simplifying setup scripts
However, because modules that use top-level await pause execution until the awaited promise resolves, it should be used carefully in performance-sensitive code.
The Self-Starter: Async IIFE#
An IIFE (pronounced “iffy”) stands for Immediate Invoked Function Expression is a function that runs as soon
as it is defined. When combined with async, it allows you to use await inside a script without having to formally
name a function and call it later. It creates a private, asynchronous execution context.
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(async () => {
try {
const data = await fetchData();
console.log("Initialization complete:", data);
} catch (err) {
console.error("Failed to start app:", err);
}
})();
Why Use It?
Top-Level Await Alternative: Useful in environments (like older Node.js versions or certain
<script>tags) where you can’t useawaitoutside of a function.Encapsulation: It keeps your variables out of the global namespace, preventing “variable pollution” and ensuring your logic doesn’t leak into the
windoworglobalobjects.Fire-and-Forget Initialization: It’s the perfect pattern for setup logic that needs to run exactly once when the script loads, such as connecting to a database or fetching initial configuration.
Async Loops and Iteration#
Avoid forEach with async code.
Incorrect
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items.forEach(async (item) => {
await process(item);
});
Correct
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for (const item of items) {
await process(item);
}
Or parallel:
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await Promise.all(items.map(process));
Async Iterators and for-await-of#
JavaScript also supports asynchronous iteration, which allows you to work with streams of data that arrive over time rather than all at once.
This pattern is built around Async Iterators, which return promises for each iteration step instead of immediate values.
The for await...of loop allows you to consume these asynchronous sequences in a clean, sequential style.
Basic Example#
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async function* generateNumbers() {
yield 1;
yield 2;
yield 3;
}
for await (const num of generateNumbers()) {
console.log(num);
}
Each iteration waits for the promise returned by the iterator to resolve before continuing.
Common Use Cases#
Async iteration is commonly used with:
Streams
Large datasets
Network responses
File processing
For example, in Node.js streams or web APIs that return chunks of data over time.
Example with Fetch Streams#
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const response = await fetch("/large-data");
const reader = response.body;
for await (const chunk of reader) {
console.log(chunk);
}
Why Async Iteration Matters#
Async iterators allow developers to process data progressively instead of waiting for an entire operation to finish. This can significantly improve performance and memory usage when dealing with large or streaming data sources.
Cancellation and Timeouts#
Promises are not cancelable by default.
Using AbortController#
Use AbortController to cancel fetches when a user navigates away, preventing memory leaks and unnecessary network usage.
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const controller = new AbortController();
fetch(url, { signal: controller.signal });
controller.abort();
This is essential for:
Search inputs
Navigation changes
Resource cleanup
Error Handling Patterns#
Global Error Handling#
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window.addEventListener("unhandledrejection", (e) => {
console.error(e.reason);
});
Retry Logic#
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async function retry(fn, retries = 3, delay = 1000) {
try {
return await fn();
} catch (e) {
if (retries === 0) throw e;
// Wait for the specified delay
await new Promise(resolve => setTimeout(resolve, delay));
// Recursive call: reduce retry count and double the delay
return retry(fn, retries - 1, delay * 2);
}
}
Sequential vs. Parallel Execution#
A common performance mistake is unnecessary sequential await usage. Async/await does not mean sequential-only.
Don’t await independent tasks one after another; it slows down your app. Use Promise.all to run them concurrently.
Sequential (Slower)#
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const a = await getUser(id);
const b = await getSettings(id);
Parallel (Faster)#
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const [a, b] = await Promise.all([
getUser(id),
getSettings(id)
]);
This runs operations in parallel while keeping readable syntax.
Use sequential execution only when one task depends on another.
Comparison Table: Which one to use?#
| Feature | Callbacks | Promises | Async/Await |
|---|---|---|---|
| Readability | Poor (Nesting) | Moderate (Chaining) | Excellent (Linear) |
| Error Handling | Manual/Difficult | .catch() |
try/catch
|
| Complexity | High for multiple tasks | Medium | Low |
When to Use What#
Callbacks: Best for low-level APIs (like Node.js
fsmodule), event listeners (element.addEventListener), or simple one-off timers.Promises: Ideal for library authors, creating composable async logic, or when you need the utility of
Promise.allandPromise.race.Async/Await: The default choice for application business logic. It makes complex operations look clean and sequential.
Pro Tip: Modern JavaScript favors
async/awaitfor clarity, but remember that it is built on top of promises. You cannot truly master one without the other.
Common Mistakes to Avoid#
The “Ghost” Promise: Forgetting to
awaita promise, which results in the code continuing before the task is finished.Silent Failures: Using async functions without a try/catch block, causing unhandled rejections that are difficult to debug.
Waterfall Slowness: Accidentally running independent async tasks in a sequence (waterfall) instead of using Promise.all to run them in parallel.
The Hybrid Mess: Mixing callbacks and promises inconsistently in the same function, leading to “Inversion of Control” bugs.
Best Practices#
Prefer
async/await: It results in a much cleaner stack trace and is significantly easier to debug than nested.then()chains.Avoid
forEachfor Async:forEachis not promise-aware. It will fire off all your async calls and finish before they resolve. Usefor...ofif you need to run tasks one-by-one, or.map()withPromise.allfor parallel execution.Handle Every Rejection: Always wrap your
awaitcalls in atry/catchblock or ensure there is a globalunhandledrejectionlistener for safety.Beware of Microtask Starvation: Because microtasks (Promises) have VIP priority, a recursive function that constantly adds new microtasks can “starve” the event loop, preventing the UI from rendering or macrotasks (like
setTimeout) from ever firing.Keep the Main Thread Clear: Offload heavy CPU-bound computation (like image processing or complex math) to Web Workers. Asynchronous code handles waiting well, but it doesn’t help with heavy lifting on a single thread.
Wrapping up#
Asynchronous JavaScript has evolved to solve real problems:
Callbacks enabled non-blocking behavior
Promises brought structure and composability
Async/await delivered clarity and maintainability
Mastering these concepts allows you to write JavaScript that scales—not just in performance, but in readability and long-term maintainability.
Understanding why async patterns exist is the key to using them correctly.
Table of contents
I hope you found this post informative and helpful. It took a lot of work to create, and I’m thrilled to finally share it with the world. Thank you for reading. 💖