The **`preventExtensions()`** method of the `Object` class in Javascript prevents objects from gaining new properties.

The process is illustrated below:

To use the `preventExtensions()` method, you will need to use the following command:

```Javascript
Object.preventExtensions(anyObj);
```

In the code shown above, `anyObj` represents an instance of the `Object` class.

<h2>Parameters</h2>

The `preventExtensions()` method takes a single mandatory parameter, i.e., the object to be made non-extensible.

<h2>Return value</h2>

The `preventExtensions()` method returns a non-extensible object. New properties can no longer be added to the object.

<h2>Example</h2>

The code below shows how the `preventExtensions()` method works in Javascript: 

// initialize object
const myObj = {};

// check if extensible
console.log("Can the object be extended? ", Object.isExtensible(myObj));

// add new property
try 
{
  Object.defineProperty(myObj, 'property1', {value: 5});
  console.log("New property added.")
}
catch (error)
{
  console.log(error.message);
}

// make object non-extensible
Object.preventExtensions(myObj);

// check if extensible
console.log("Can the object be extended? ", Object.isExtensible(myObj));

// add new property
try 
{
  Object.defineProperty(myObj, 'property2', {value: 10});
  console.log("New property added.")
}
catch (error)
{
  console.log(error.message);
}

<h2>Explanation</h2>

First, an empty object `myObj` is initialized. The `isExtensible()` method is called to check if new properties can be added to `myObj`. Since objects are extensible by default, the `isExtensible()` method returns `true`.

The `defineProperty()` method in line $10$ adds a new property to `myObj`. Since the object is extensible, the `defineProperty()` method does not throw an error.

Next, the `preventExtensions()` method in line $19$ proceeds to make `myObj` a non-extensible object. As a result, new properties can no longer be added to it. The `isExtensible()` method in line $22$ returns `false`, which confirms that `myObj` is non-extensible.

Finally, the `defineProperty()` method in line $27$ attempts to add a new property to `myObj`. However, `myObj` is now a non-extensible object, so an error is thrown. The error triggers the `catch` branch of the `try-catch` block, and the error message is output.

What is the Object.preventExtensions() method in Javascript?

The `preventExtensions()` method prevents objects from gaining new properties and returns the non-extensible object.