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Handlers for Operators

Each method recognized in the handler for Proxy has a corresponding method with the same name in Reflect. Implementing such a Proxy method affects the corresponding Reflect method.

.get()

A trap for getting property values, eg. o[p], o.p.

.set()

A trap for setting property values, eg. o.p = X.


var handler = {
  get: function(target, prop, receiver) {
    return target[prop] * 2;
  },
  set: function(target, prop, value, receiver) {
    target[prop] = value + 100;
    return true;
  }
};

var o = { a: 3 };
var p = new Proxy(o, handler);

console.log(o.a);
console.log(p.a);

p.b = p.a; // (changing p changes o)
console.log(o.b);
console.log(p.b);

o.b = o.a; // (changing o also changes p)
console.log(o.b);
console.log(p.b);

var p = new Proxy(o, { // (restore to default behaviours for these two handlers)
  get: Reflect.get,
  set: Reflect.set
});

p.b = p.a;
console.log(p.b);

3 6 106 212 3 6 3

.has()

A trap for the in operator.


var p = new Proxy({}, {
  has: function(target, prop) {
    return true;
  }
});

console.log('a' in p);
console.log(Reflect.has(p, 'a'));
// … (affects the corresponding Reflect method)

p = new Proxy({}, Reflect); // (restores all default behaviours)
console.log('a' in p);

true true false

.deleteProperty()

A trap for the delete operator.


var p = new Proxy({ a: 3 }, {
  deleteProperty: function(target, prop) {
    if (prop in target) {
      delete target[prop];
      console.log('property removed: ' + prop);
      return true;
    } else {
      console.log('property not found: ' + prop);
      return false;
    }
  }
});

delete p.a;
delete p.a;

property removed: a property not found: a