Why should you care?

JavaScript uses callbacks for events and async work before promises became standard.

See it live — copy this example

Paste into an HTML file or the browser console (F12). Use Run below when the live editor is available.

function greet(name, done) {
  console.log("Hello, " + name);
  done();
}
greet("Sam", () => console.log("Finished"));

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• greet runs synchronously, then calls done().
• The arrow function is the callback.

Practice next

1. Run greet with the done callback.
2. Use setTimeout(() => console.log("Later"), 1000).
3. Nest two callbacks and notice callback hell.
4. Add an onError callback parameter to a fake fetch helper.
5. Simulate async order: step1(cb) then step2(cb) with nested calls.

Remember

Pass functions as arguments Run later on events or timers Promises/async often replace nested callbacks

ScriptVerse file upload

A legacy upload widget calls onProgress(percent) and onComplete(url) callbacks during multipart upload.

Outcome: Callbacks still appear in browser APIs and older libraries — you must read them fluently.