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JavaScript | handler.isExtensible() Method

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  • Last Updated : 28 Sep, 2021

The handler.isExtensible() method in JavaScript is a trap for Object.isExtensible() method and it returns a boolean value.
Syntax: 
 

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

Parameters: This method accept single parameter as mentioned above and described below: 
 

  • target: This parameter is the target object.

Return value: This method returns a Boolean value.
Below examples illustrate the handler.isExtensible() method in JavaScript:
Example 1: 
 

javascript




<script>
const monster1 = {
  canEvolve: true
};
 
const handler1 = {
  isExtensible(target) {
    return Reflect.isExtensible(target);
  },
  preventExtensions(target) {
    target.canEvolve = false;
    return Reflect.preventExtensions(target);
  }
};
 
const proxy1 = new Proxy(monster1, handler1);
document.writeln(Object.isExtensible(proxy1));
document.writeln("<br>");
document.writeln(monster1.canEvolve);
document.writeln("<br>");
document.writeln(Object.preventExtensions(proxy1));
document.writeln("<br>");
document.writeln(Object.isExtensible(proxy1));
document.writeln("<br>");
document.writeln(monster1.canEvolve); 
</script>

Output: 
 

true
true
[object Object]
false
false

Example 2: 
 

javascript




<script>
const p = new Proxy({}, {
  isExtensible: function(target) {
    console.log('isExtensible method');
    return true;
  }
});
 
console.log(Object.isExtensible(p));
 
var a = { 
  canEvolve: true 
}; 
var b = {  
  isExtensible(target) { 
    return true
  }, 
   }; 
const proxy1 = new Proxy(a, b); 
console.log(Object.isExtensible(proxy1));
</script>

Output: 
 

"isExtensible method"
true
true

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

  • Google Chrome 49 and above
  • Edge 12 and above
  • Firefox 31 and above
  • Opera 36 and above
  • Safari 10 and above

 


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