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:
- Target: This parameter holds the target object.
- property: This parameter is the name or Symbol of the property whose description is going to be retrieved..
- descriptor: This parameter is the descriptor for the property being defined or modified.
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.
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' , {});
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Output:
"Type : String" "in defineProperty"
Example 2: In this example, we will see the use of the handler.defineProperty() method in 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' );
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Output:
Error: Invalid attempt to define private "_propt" property
Supported Browsers: The browsers supported by handler.defineProperty() method are listed below:
- Google Chrome 49 and above
- Firefox 18 and above
- Opera 36 and above
- Safari 10 and above
- Edge 12 and above
We have a complete list of Javascript Proxy/handler methods, to check those go through the Javascript Proxy/handler Reference article.