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JavaScript75 min total · 15 parts

JavaScript Core Concepts: Scope, Closures, `this`, and the Event Loop

Contents — Part 9 of 15: Synchronous vs. Asynchronous Execution
  1. 1.Overview
  2. 2.Scope: Global, Function, and Block
  3. 3.var vs. let vs. const: A Direct Comparison
  4. 4.Hoisting and the Temporal Dead Zone
  5. 5.Closures: A Function Remembers Where It Was Created
  6. 6.Practical Closure Patterns: Memoization, Debounce, and the Module Pattern
  7. 7.this: How It Gets Bound
  8. 8.The Call Stack and Stack Overflows
  9. 9.Synchronous vs. Asynchronous Execution
  10. 10.Promises: Representing a Future Value
  11. 11.async/await: Promises With Synchronous-Looking Syntax
  12. 12.The Event Loop: Microtasks vs. Macrotasks
  13. 13.Where Rendering Fits In
  14. 14.setTimeout, setInterval, and Their Gotchas
  15. 15.Common Mistakes Worth Remembering
Part 9 of 15 · ~1 min

Synchronous vs. Asynchronous Execution

"Asynchronous" in JavaScript doesn't mean "runs on another thread" — it means "runs later, after the current synchronous code has finished, scheduled by the browser/runtime rather than by the normal call stack."

console.log("1");
setTimeout(() => console.log("2"), 0);
console.log("3");
// Output: 1, 3, 2 — even a 0ms timeout is scheduled for LATER, never immediately

setTimeout(fn, 0) doesn't mean "run in 0 milliseconds" — it means "run as soon as possible after the current synchronous code finishes and the stack is empty," which is a fundamentally different guarantee. This single fact explains a large share of async confusion: synchronous code always finishes completely before any scheduled callback gets a chance to run, no matter how small its delay.

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The Call Stack and Stack Overflows
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Promises: Representing a Future Value