JavaScript Handler preventExtensions() Method

JavaScript handler.preventExtensions() method in JavaScript is a trap for Object.preventExtensions() method. New properties cannot be added to an object when extensions are disabled. It returns a boolean value.

Syntax:

const p = new Proxy(target, {
      preventExtensions: function(target) {
  }
});

Parameters: This method accepts a single parameter target that holds the target object.

Return value: This method returns a boolean value.

Below examples illustrate the handler.preventExtensions() method in JavaScript:

Example 1: In this example, we will see the use of the handler.preventExtensions() method in JavaScript.

javascript

const p = new Proxy({}, {
    preventExtensions: function (target) {
        console.log('preventExtensions()');
        console.log(Object.preventExtensions(target));
        return true;
    }
});

console.log(Object.preventExtensions(p));

let x = {
    first: false
};
let y = {
    preventExtensions(target) {
        target.canEvolve = false;
        Object.preventExtensions(target);
        return true;
    }
};
let proxy = new Proxy(x, y);
console.log(x.first);
Object.preventExtensions(proxy);
console.log(x.first);

Output:

true
[object Object]
false
true

Example 2: In this example, we will see the use of the handler.preventExtensions() method in JavaScript.

javascript

const p = new Proxy({}, {
    preventExtensions: function (target) {
        console.log('preventExtensions()');
        console.log(Object.preventExtensions(target));
        return true;
    }
});

console.log(Object.preventExtensions(p));

let x = {
    first: false
};
let y = {
    preventExtensions(target) {
        target.canEvolve = false;
        Object.preventExtensions(target);
        return true;
    }
};
let proxy = new Proxy(x, y);
console.log(x.first);
Object.preventExtensions(proxy);
console.log(x.first);

Output:

preventExtensions()
[object Object]
[object Object]
false
false

Supported Browsers: The browsers supported by handler.preventExtensions() method are listed below: 

• Google Chrome 49 and above
• Edge 12 and above
• Firefox 22 and above
• Opera 36 and above
• Safari 10 and above

We have a complete list of Javascript Proxy/handler methods, to check those go through the Javascript Proxy/handler Reference article.