Async and Promises

JavaScript is single-threaded. Long operations (network, timers) are handled asynchronously so the UI does not block.

Callbacks (legacy)

javascript
setTimeout(() => console.log(\"Later\"), 1000);

Nested callbacks become hard to read (callback hell).

Promises

A Promise represents a value (or error) that will be available later:

javascript
const p = fetch(\"/api/data\")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

then runs when the promise resolves; catch when it rejects.

async/await

Syntactic sugar over promises. Use in an async function:

javascript
async function loadData() {
  try {
    const res = await fetch(\"/api/data\");
    const data = await res.json();
    return data;
  } catch (err) {
    console.error(err);
  }
}

await pauses the function until the promise settles. Always use try/catch around await if you need to handle errors.