JavaScript handler.preventExtensions() Method

The handler.preventExtensions() method is used to trap the Object.preventExtensions method. When extensions are prevented on an object, new properties can't be added.

Syntax

Parameters

target: The target object.

Return value

Return a Boolean value.

Browser Support

Chrome	49
Edge	12
Firefox	22
Opera	36

Example 1

<script>
const proxy = new Proxy({}, {
  preventExtensions: function(target) {
   Object.preventExtensions(target);
    return !Object.isExtensible(target);
  }
});
document.writeln(Object.isExtensible(proxy));
//expected output:true
</script>

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Output:

true

Example 2

<script>
const  proxy1 = new Proxy({}, {
  preventExtensions: function(target) {
  

    Object.preventExtensions(target);
    return !Object.isExtensible(target);
  }
});
document.writeln(Object.isExtensible(proxy1));
//expected output: true 
document.writeln('<br/>');
document.writeln(Object.preventExtensions(proxy1));
// Object.preventExtensions prevent of object to access.
//expected output: [object Object] 
document.writeln('<br/>');
document.writeln(Object.isExtensible(proxy1));
//expected output: false
</script>

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Output:

true 
[object Object] 
false

Example 3

<script>
var x = {
  first: false
};
var y ={
  preventExtensions(target) {
    target.canEvolve = false;
    Object.preventExtensions(target);
    return true;
  }
};
var proxy = new Proxy(x, y);
document.writeln(x.first);
// expected output: false
Object.preventExtensions(proxy);
document.writeln("<br/>");
document.writeln(x.first);
// expected output: false
</script>

Test it Now

Output: