JavaScript handler preventExtensions() Method

by Online Tutorials Library July 22, 2022

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>  

Test it Now

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:

false   false

Next TopicJavaScript handler

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JavaScript handler preventExtensions() Method

JavaScript handler.preventExtensions() Method

Syntax

Parameters

Return value

Browser Support

Example 1

Example 2

Example 3

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