JavaScript ProxyHandler : Object

The ProxyHandler is used by Proxy to intercept and modify the behavior of the proxied object. You may use any JavaScript object as a ProxyHandler. If any of the following methods are not provided, the Proxy will use the default behavior for that method.

ECMAScript 2015

Instance Methods

apply(target : Function, thisArg : Object, parameters : Array) : Object

Called when proxy(...), proxy.apply(thisArg, parameters), or proxy.call(thisArg, ...parameters) are called.

Example:

var x = function(arg1, arg2) {
  console.log('in x(' + arg1 + ', ' + arg2 + ')');
};

var proxy = new Proxy(x, {
  apply: function(target, thisArg, parameters) {
    console.log('in apply');

    return target.apply(thisArg, parameters);
  }
});

proxy('direct1', 'direct2');
proxy.apply(null, ['apply1', 'apply2']);
proxy.call(null, 'call1', 'call2');

Run

Results:

construct(target : Function, parameters : Array) : Object

Called when new target(...parameters) is called.

Example:

var Button = function(text) {
  this.text = text;
};

var ButtonProxy = new Proxy(Button, {
  construct: function(target, parameters) {
    console.log('in construct');

    var button = Object.create(target.prototype);
    target.apply(button, parameters);
    return button;
  }
});

var button = new ButtonProxy('OK');
console.log(button.text);

Run

Results:

defineProperty(target : Object, name : String, propertyDescriptor : PropertyDescriptor) : undefined

Called when Object.defineProperty() is called on the proxy.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  defineProperty: function(target, name, propertyDescriptor) {
    console.log('in defineProperty');

    return Object.defineProperty(target, name, propertyDescriptor);
  }
});

Object.defineProperty(proxy, 'bar', { value: 2, 
                                      writable: true,
                                      enumerable: false,
                                      configurable: true } );

Run

Results:

deleteProperty(target : Object, name : String) : Boolean

Called when delete proxy[name] is called. Return true if the delete was successful.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  deleteProperty: function(target, name) {
    console.log('in deleteProperty');
 
    return delete target[name];
  }
});

delete proxy.foo;

console.log(proxy.foo);
console.log(x.foo);

Run

Results:

enumerate(target : Object) : Array<String>

get(target : Object, name : String, proxy : Proxy) : Object

Called when reading the name property of the target of the proxy. The return value is used as the value of the property.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  get: function(target, name, proxy) {
    console.log('in get');

    var value = target[name];
    if (typeof value === 'string') {
      value = value.toUpperCase();
    }
    return value;
  }
});

console.log(proxy.foo);
console.log(x.foo); // value stored on x is unmodified

Run

Results:

getOwnPropertyDescriptor(target : Object, name : String) : PropertyDescriptor

Called when Object.getOwnPropertyDescriptor() is called on the Proxy.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  getOwnPropertyDescriptor: function(target, name) {
    console.log('in getOwnPropertyDescriptor');

    return Object.getOwnPropertyDescriptor(target, name);
  }
});

console.dir(Object.getOwnPropertyDescriptor(proxy, 'foo'));

Run

Results:

has(target : Object, name : String) : Boolean

Called when name in proxy is called.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  has: function(target, name) {
    console.log('in has');

    return name in target;
  }
});

console.log('foo' in proxy);

Run

Results:

isExtensible(target : Object) : Boolean

Called when checking to see if an Object is extensible.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  isExtensible: function(target) {
    console.log('in isExtensible');

    return Object.isExtensible(target);
  }
});

console.log(Object.isExtensible(proxy));
Object.preventExtensions(proxy);
console.log(Object.isExtensible(proxy));

Run

Results:

ownKeys(target : Object) : Array<String>

Called when Object.keys(), Object.getOwnPropertyNames(), or Object.getOwnPropertySymbols() are called on the proxy.

Example:

var x = { foo: 1 };
Object.defineProperty(x, 'bar', { value: 2, 
                                  writable: true,
                                  enumerable: false,
                                  configurable: true } );

var proxy = new Proxy(x, {
  ownKeys: function(target, a, b) {
    console.log('in ownKeys');

    var names = Object.getOwnPropertyNames(target);
    var symbols = Object.getOwnPropertySymbols(target);
    return names.concat(symbols);
  }
});

console.log(Object.keys(proxy));
console.log(Object.getOwnPropertyNames(proxy));
console.log(Object.getOwnPropertySymbols(proxy));

Run

Results:

preventExtensions(target : Object) : Boolean

Called when Object.preventExtensions() is called on the Proxy. Return true if preventExtensions succeeded.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  preventExtensions: function(target) {
    console.log('in preventExtensions');

    Object.preventExtensions(target);
    return !Object.isExtensible(target);
  }
});

console.log(Object.isExtensible(proxy));
console.log(Object.preventExtensions(proxy));
console.log(Object.isExtensible(proxy));

Run

Results:

set(target : Object, name : String, value : Object, proxy : Proxy) : Object

Called when setting the name property of the target of the proxy.

Example:

var x = { foo: 1 };

var proxy = new Proxy(x, {
  set: function(target, name, value, proxy) {
    console.log('in set');

    target[name] = value.toUpperCase();
  }
});

proxy.foo = 'bar';

console.log(proxy.foo);
console.log(x.foo); // value stored on x is uppercase

Run

Results:

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