Node.js – Callback Pattern & Callback Functions | Node.js Tutorial

📌 What is Callback?

What is a Callback?

A callback is a function passed as an argument to another function. It allows a function to call another function and can run after another function has finished. Callback functions can also be returned as values from other functions.

This is a fundamental pattern in Node.js to handle asynchronous operations.

📌 Example: Simple Callback

Compare easy understanding of callback and higher-order function:

Node.js Environment

RUNTIME ACTIVE

function show1(name) {
  console.log(`Hello ${name}`);
}

function show2(showName) {
  const name = "Coding Point";
  showName(name);
}

show2(show1);

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⚡ Types of Callbacks

Synchronous Callbacks
Asynchronous Callbacks

✅ Synchronous Callbacks

A callback function which is executed immediately is called a synchronous callback.

Example: see the previous 'Simple Callback' example.

⏳ Asynchronous Callbacks

An asynchronous callback is used to continue or resume code execution after an asynchronous operation has completed.

Example use cases: fetching 50,000 rows from a database or reading a file with 50,000 lines. The callback function executes after the operation completes.

🌐 Asynchronous Callbacks – Browser Example

Node.js Environment

RUNTIME ACTIVE

function callback() {
  document.getElementById("demo").innerHTML = "Hello World";
}
document.getElementById("btn").addEventListener("click", callback);

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🌐 Asynchronous Callbacks – jQuery Example

Node.js Environment

RUNTIME ACTIVE

$.get("url", function(data) {
  $(".result").html(data);
  alert("Load was performed.");
});

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📂 Asynchronous Callbacks in Node.js

Node.js uses asynchronous callbacks to prevent blocking of execution.

Node.js Environment

RUNTIME ACTIVE

const fs = require("fs");

fs.readFile("demo.txt", (err, data) => {
  if (err) throw err;
  console.log(data.toString());
});

Terminal Output

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