Monkey Patch In JavaScript

In JavaScript, a "monkey patch" is a technique that involves modifying or adding functionality to an existing object or function at runtime without changing the original source code. This approach can be useful when the original code is unavailable or cannot be modified directly. This article will explore what monkey patching is, how it works, and when it can be used.

What is Monkey Patching?

Monkey patching is a technique used in programming languages that support dynamic typing and runtime code modification. In JavaScript, this involves modifying an object or function's prototype and adding or overriding methods, properties, or attributes at runtime.

Monkey patching can add functionality to existing objects, change the behavior of existing methods, or patch a bug without modifying the original code. It is important to note that monkey patching can be a double-edged sword, and care should be taken when applying it.

How Does Monkey Patching Work?

Monkey patching in JavaScript is achieved by modifying an object or function's prototype or adding new properties and methods to it. The prototype is a special object used as a blueprint for creating new instances of an object. It contains a set of properties and methods inherited by all instances of the object.

For example, to add a new method to an existing object, we can modify its prototype as follows,

// Define an object 
const myObj = {
    foo: () => {
        console.log("Original foo");
// Add a new method to the object's prototype = () => {
    console.log("test bar");
// Call the new method;
// Output: "test bar"

In the example above, we defined an object myObj with an existing method foo. We then added a new method bar to the object's prototype using the __proto__ property. Finally, we called the new method, which outputs a "test bar" to the console.

When to Use Monkey Patching

Monkey patching can be a useful technique in certain situations, such as:

  • Fixing bugs in third-party code: If you encounter a bug in a library or framework that you cannot modify directly, you can use monkey patching to patch the bug at runtime.
  • Adding missing functionality: If you are working with an object or function missing a critical feature, you can add it at runtime using monkey patching.
  • Testing: In some cases, modifying the original code for testing purposes may be difficult or impractical. Monkey patching can modify behavior for testing without changing the original code.

However, monkey patching can also have drawbacks, such as making code harder to read and maintain, increasing the risk of unexpected behavior, and introducing security vulnerabilities. Therefore, it should be used judiciously and with caution.


Monkey patching is a powerful technique that can be used to modify or add functionality to an object or function at runtime in JavaScript. It can be a valuable tool for fixing bugs, adding missing functionality, and testing, but it can also have drawbacks and should be used carefully. Understanding how monkey patching works and when to use it can help you write more flexible and robust code.