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Asynchronous Code

Asynchronous lines, like:

setTimeout()

fetch()

async

Promise.then()

event listeners

…are not executed immediately in the same step. Instead, they’re deferred, often pushed to:

The microtask queue (e.g., .then(), await)

The macrotask queue (e.g., setTimeout, I/O)

These deferred tasks only run after the synchronous code and the current call stack is empty.

function run() {
  console.log("1"); // sync

  setTimeout(() => console.log("2"), 0); // async (macrotask)
  
  Promise.resolve().then(() => console.log("3")); // async (microtask)

  console.log("4"); // sync
}

run();

What will the output be?

Asynchronous lines are deferred, so they don’t block the synchronous ones — but how they're deferred (microtask vs. macrotask) affects when they run afterward.

Want a quick memory trick?

"Sync first, microtask next, macrotask last."