Closures allow inner functions to remember variables from their outer scope even after the outer function finishes, enabling data privacy and function factories. 2. The event loop is not part of JavaScript but of the runtime environment, coordinating between the call stack, callback queue, and asynchronous browser APIs to manage execution order. 3. In the loop with setTimeout example, using var leads to all closures referencing the same i (resulting in 4, 4, 4), which can be fixed by using block-scoped let or creating individual closures with an IIFE to capture each value correctly. Understanding closures and the event loop clarifies how JavaScript handles scope, memory, and asynchronous operations, making seemingly confusing behavior predictable and logical.
JavaScript can seem tricky when you're first learning about closures and the event loop — two of the most powerful yet misunderstood concepts in the language. They’re not magic, though. Once you understand how they work, a lot of "weird" JavaScript behavior starts to make sense. Let’s break them down in plain terms.
What Are Closures? (And Why They Matter)
A closure is a function that remembers the variables from the scope in which it was created — even after that scope has finished running.
Here’s a simple example:
function outer() {
let name = "Alice";
function inner() {
console.log(name); // inner "remembers" `name`
}
return inner;
}
const func = outer();
func(); // logs "Alice"Even though outer() has already finished executing, the inner function still has access to the name variable. That’s a closure.
Key points:
- A closure is created every time a function is defined inside another function.
- It allows the inner function to access outer function variables.
- Closures are the foundation for patterns like data privacy, function factories, and callback handlers.
Practical use case:
function createCounter() {
let count = 0;
return function() {
count ;
console.log(count);
};
}
const counter = createCounter();
counter(); // 1
counter(); // 2The count variable is "private" — no external code can touch it. Only the returned function can modify it. This is closure in action.
How the Event Loop Works (It’s Not Part of JavaScript)
Here’s a common misconception: the event loop is not part of the JavaScript engine. It’s part of the JavaScript runtime environment — like your browser or Node.js.
JavaScript itself is single-threaded: it can only do one thing at a time. But the browser can do more (like handle clicks, timers, HTTP requests). The event loop is what coordinates all this.
The JavaScript Execution Model
When you run code, JavaScript uses:
- Call stack – tracks function calls.
- Callback queue – holds async callbacks (like from
setTimeoutor click events). - Event loop – checks if the call stack is empty, and if so, pushes a callback from the queue onto the stack.
Let’s see it in action:
console.log("1");
setTimeout(() => {
console.log("2");
}, 0);
console.log("3");You might expect: 1, 2, 3
But you get: 1, 3, 2
Why?
console.log("1")runs immediately.setTimeoutis async — it’s handed off to the browser API, which waits 0ms, then puts the callback in the callback queue.console.log("3")runs next (synchronous code finishes).- Only after the call stack is empty does the event loop pick up the queued callback and run
console.log("2").
So: synchronous code always runs before async callbacks, no matter the timeout delay.
Putting Closures and the Event Loop Together
A classic gotcha combines closures and the event loop:
for (var i = 1; i <= 3; i ) {
setTimeout(() => {
console.log(i);
}, 100);
}You might expect: 1, 2, 3
But you get: 4, 4, 4
Why?
varis function-scoped and shared across iterations.- By the time the
setTimeoutcallbacks run, the loop has finished andi === 4. - All closures reference the same
i.
Fix 1: Use let (block-scoped)
for (let i = 1; i <= 3; i ) {
setTimeout(() => {
console.log(i);
}, 100);
}
// Logs 1, 2, 3 — because each `i` is scoped to the blockFix 2: Create a closure with an IIFE
for (var i = 1; i <= 3; i ) {
(function(num) {
setTimeout(() => {
console.log(num);
}, 100);
})(i);
}Now each timeout gets its own copy of i via the closure.
Closures and the event loop aren’t just interview trivia — they shape how real apps behave. Closures let you encapsulate data and create reusable functions with memory. The event loop ensures your app stays responsive, even when dealing with delays or user input.
Understanding them removes the mystery from async code, timers, and state management in JavaScript.
Basically, closures are about scope and memory, and the event loop is about timing and coordination. Get these, and a lot of JavaScript starts to click.
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