JavaScript | handler.preventExtensions() Method
The handler.preventExtensions() method in JavaScript is a trap for Object.preventExtensions() method and it returns a boolean value.
Syntax:
const p = new Proxy(target, {
preventExtensions: function(target) {
}
});Parameters: This method accepts single parameter target which holds the target object.
Return value: This method returns a boolean value.
Below examples illustrate the handler.preventExtensions() method in JavaScript:
Example 1:
javascript
<script>const monster1 = { canEvolve: true};const handler1 = { preventExtensions(target) { target.canEvolve = false; Object.preventExtensions(target); return true; }};const proxy1 = new Proxy(monster1, handler1);document.writeln(monster1.canEvolve);document.writeln("<br>");document.writeln(Object.preventExtensions(proxy1));document.writeln("<br>");document.writeln(monster1.canEvolve);document.writeln("<br>");const proxy = new Proxy({}, { preventExtensions: function(target) { Object.preventExtensions(target); return !Object.isExtensible(target); } }); document.writeln(Object.isExtensible(proxy));</script> |
Output:
true [object Object] false true
Example 2:
javascript
<script>const p = new Proxy({}, { preventExtensions: function(target) { document.writeln('preventExtensions()'+"<br>"); document.writeln(Object.preventExtensions(target)+"<br>"); return true; }});document.writeln(Object.preventExtensions(p)+"<br>");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); Object.preventExtensions(proxy); document.writeln("<br/>"); document.writeln(x.first);</script> |
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
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