Last Updated : 12 Jul, 2025
JavaScript handler.defineProperty() method in Javascript is used to define the new properties and to modify the existing properties directly on an object. It is a trap for Object.defineProper().
Syntax:
const p = new Proxy(target, {
defineProperty: function(target, property, descriptor) {
}
});
Parameters: This method accepts three parameters as mentioned above and described below:
Return value: This method returns a Boolean value which is used to indicate if the property is successfully defined.
Below example illustrates the handler.defineProperty() method in JavaScript:
Example 1: In this example, we will see the use of the handler.defineProperty() method in JavaScript.
javascript
const p = new Proxy({}, {
defineProperty: function (target, prop, descriptor) {
console.log('Type : ' + prop);
return true;
}
});
const desc = { configurable: true, enumerable: true, value: 10 };
Object.defineProperty(p, 'String', desc);
let xyz = {};
let proxy = new Proxy(xyz, {
defineProperty: function (target, name, propertyDescriptor) {
console.log('in defineProperty');
return Object.defineProperty(target, name, propertyDescriptor);
}
});
Object.defineProperty(proxy, 'bar', {});
Output:
"Type : String" "in defineProperty"
Example 2: In this example, we will see the use of the handler.defineProperty() method in JavaScript.
javascript
const handler1 = {
defineProperty(target, key, descriptor) {
invariant(key, 'define');
return true;
}
};
function invariant(key, action) {
if (key[0] === '_') {
throw new Error(
`Invalid attempt to ${action} private "${key}" property`);
}
}
const monster1 = {};
const proxy1 = new Proxy(monster1, handler1);
console.log(proxy1._propt = 'Handler defineProperty');
Output:
Error: Invalid attempt to define private "_propt" property
Supported Browsers: The browsers supported by handler.defineProperty() method are listed below:
We have a complete list of Javascript Proxy/handler methods, to check those go through the Javascript Proxy/handler Reference article.
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