Callback Functions in JavaScript 🔄

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By Ronak Wanjari, Intern Devsync

While working on async tasks at DevSync, I kept hearing the word “callback” — especially in functions like setTimeout(), APIs, and file operations.

So here’s a simple breakdown of callback functions — with examples you’ll actually use.

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🔹 What is a Callback?

A callback function is a function passed as an argument to another function — and it’s called later when something finishes.

function greet(name, callback) {
  console.log("Hi " + name);
  callback();
}

function done() {
  console.log("Greeting complete!");
}

greet("DevSync", done);

👉 Output:

Hi DevSync  
Greeting complete!

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🔹 Real Example: setTimeout uses Callback

setTimeout(() => {
  console.log("Executed after 2 seconds");
}, 2000);

Here, the arrow function is a callback — it runs after the timeout.

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🔹 Synchronous vs Asynchronous Callbacks

🟢 Synchronous:

function calculate(a, b, callback) {
  let result = a + b;
  callback(result);
}

calculate(5, 3, (sum) => {
  console.log("Sum is", sum); // Sum is 8
});

🟠 Asynchronous:

function fetchData(callback) {
  setTimeout(() => {
    callback("Data loaded");
  }, 1000);
}

fetchData((data) => {
  console.log(data); // Data loaded
});

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🔹 Why Use Callbacks?

• Handle async operations (API calls, timers, I/O).

• Keep code modular and reusable.

• Avoid repeating logic.

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⚠️ Callback Hell

Too many nested callbacks look messy:

doA(() => {
  doB(() => {
    doC(() => {
      console.log("All done!");
    });
  });
});

🧠 Solution? Use Promises or async/await (but that's another blog 😉)

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🔚 Conclusion

At Devsync , I used callbacks daily — from button click handlers to API responses. They're core to writing good JavaScript, especially when things aren't instant (like user input or data fetching).

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✅ Quick Recap:

• A callback is a function passed to another function.

• Used a lot in async code (like setTimeout, APIs).

• Avoid nesting too deep — use Promises later!

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📌 Want more JavaScript breakdowns from a dev intern’s POV?
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🔗 Devsync

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