What is Callback?

A Callback in Node.js is an asynchronous equivalent for a function. It is a special type of function passed as an argument to another function. Node.js makes heavy use of callbacks. Callbacks help us make asynchronous calls. All the APIs of Node are written in such a way that they support callbacks.

Programming instructions are executed synchronously by default. If one of the instructions in a program is expected to perform a lengthy process, the main thread of execution gets blocked. The subsequent instructions can be executed only after the current I/O is complete. This is where callbacks come in to the picture.

The callback is called when the function that contains the callback as an argument completes its execution, and allows the code in the callback to run in the meantime. This makes Node.js highly scalable, as it can process a high number of requests without waiting for any function to return results.

The syntax of implementing callback in Node.js is as follows −

function function_name(argument, function (callback_argument){
   // callback body 
})

The setTimeout() function in Node.js is a typical example of callback. The following code calls the asynchronous setTimeout() method, which waits for 1000 milliseconds, but doesn”t block the thread. Instead, the subsequent Hello World message, followed by the timed message.

Example

setTimeout(function () { 
   console.log(''This prints after 1000 ms''); 
}, 1000);

console.log("Hello World");

Output

Hello World
This prints after 1000 ms

Blocking Code Example

To understand the callback feature, save the following text as input.txt file.

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The following code reads the file synchronously with the help of readFileSync() function in fs module. Since the operation is synchronous, it blocks the execution of the rest of the code.

var fs = require("fs");
var data = fs.readFileSync(''input.txt'');

console.log(data.toString());

let i = 1;
while (i <=5) {
  console.log("The number is " + i);
  i++;
}

The output shows that Node.js reads the file, displays its contents. Only after this, the following loop that prints numbers 1 to 5 is executed.

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The number is 1
The number is 2
The number is 3
The number is 4
The number is 5

Non-Blocking Code Example

We use the same input.txt file in the following code to demonstrate the use of callback.

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The ReadFile() function in fs module is provided a callback function. The two arguments passed to the callback are error and the return value of ReadFile() function itself. The callback is invoked when ReadFile() finishes by returning either error or file contents. While the file read operation is inprocess, Node.js asynchronously runs the subsequent loop.

var fs = require("fs");
fs.readFile(''input.txt'', function (err, data) {
   if (err) return console.error(err);
   console.log(data.toString());
});

let i = 1;
while (i <=5) {
  console.log("The number is " + i);
  i++;
}

Output

The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
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Callback as Arrow function

You can also assign an arrow function as a callback argument. Arrow function in JavaScript is an anonymous function. It is also called as lambda function. The syntax of using arrow function as Node.js callback is as follows −

function function_name(argument, (callback_argument) => { 
   // callback body 
})

It was introduced in ES6 version of JavaScript. Let us replace the callback in the above example with an arrow function.

var fs = require("fs");

fs.readFile(''input.txt'',  (err, data) => {
   if (err) return console.error(err);
   console.log(data.toString());
});

let i = 1;
while (i <=5) {
  console.log("The number is " + i);
  i++;
}

The above code produces a similar output as the previous example.