Monkey Patching in Node.js? Update code at runtime in Node.js


In this blog post, we will look at what Monkey patching is, what are the pros and cons of it, and also an example that illustrates this technique.

What is Monkey patching?

Monkey patching is a technique for dynamic modification of any module or function. By using Monkey patching one can add or modify the default behavior of a piece of code at runtime without changing its original source code.

When to use Monkey patching?

Imagine you as a system administrator, and you want to check every database query which has been made to your database. If you have a web application which does some system calls and you want to intercept what arguments were passed in the system call. Therefore one basic use case of monkey patching is to sniffing the arguments of API call.

With help of monkey patching one can modify the arguments as well and can change the default behaviour of the API.

Lets create a sample program which intercept the child_process module’s API.

const cp = require('child_process');

//logic to update default exec() API
let original_function = cp.exec
cp.exec = function(){
    console.log("Intecepted arguments are:",arguments);

cp.exec('ls -lrt', (error, stdout, stderr) => {
    if (error) {
      console.error(`exec error: ${error}`); 

Output of the above code is:

% node demo.js
Intecepted arguments are: [Arguments] { '0': 'ls -lrt', '1': [Function] }

As per shown in example code, We have replaced the actual function definition of child_process.exec() to our own version of child_process.exec(). In the modified version of exec() method call, We have added a log to print all the arguments passed to the exec() API call. After that the original function call was done by using apply() function.


In this post, We have covered what Monkey patching is and how it can be used in JavaScript. Hope you got the idea behind the monkey patching. Let’s hack the programming language. Have fun

#HappyCoding # #TopCode

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1 comment

  • really helpful blog

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