Understanding the Event Loop

The event loop coordinates the call stack and queued callbacks. Promise reactions enter the microtask queue; timer callbacks are scheduled later through host tasks. Their ordering matters when diagnosing UI behavior and asynchronous tests.

Reading the code: A and B run on the initial stack. The Promise reaction is processed as a microtask after that stack empties. The zero-delay timer is eligible only on a later task turn; it prints last.

Keep the boundary clear: setTimeout(fn,0) is not a promise that fn will run immediately or at an exact wall-clock time.

Save the code as event-loop.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

console.log("A");
setTimeout(() => console.log("timer"), 0);
Promise.resolve().then(() => console.log("microtask"));
console.log("B");

Verified output

A
B
microtask
timer

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable event-loop.ts; run: node event-loop.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

A and B run on the initial stack. The Promise reaction is processed as a microtask after that stack empties. The zero-delay timer is eligible only on a later task turn; it prints last.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

setTimeout(fn,0) is not a promise that fn will run immediately or at an exact wall-clock time.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Add a second Promise.resolve().then(...) and predict its position before the timer.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

The event loop coordinates the call stack and queued callbacks.

Try the practice change and re-run the exact program to test your understanding.