handler.set()

The handler.set() method is a trap for setting a property value.

Syntax

const p = new Proxy(target, {
  set: function(target, property, value, receiver) {
  }
});

Parameters

The following parameters are passed to the set() method. this is bound to the handler.

target

The target object.

property

The name or Symbol of the property to set.

value

The new value of the property to set.

receiver

The object to which the assignment was originally directed. This is usually the proxy itself. But a set() handler can also be called indirectly, via the prototype chain or various other ways.

For example, suppose a script does obj.name = "jen", and obj is not a proxy, and has no own property .name, but it has a proxy on its prototype chain. That proxy's set() handler will be called, and obj will be passed as the receiver.

Return value

The set() method should return a boolean value.

• Return true to indicate that assignment succeeded.
• If the set() method returns false, and the assignment happened in strict-mode code, a TypeError will be thrown.

Description

The handler.set() method is a trap for setting property value.

Interceptions

This trap can intercept these operations:

• Property assignment: proxy[foo] = bar and proxy.foo = bar
• Inherited property assignment: Object.create(proxy)[foo] = bar
• Reflect.set()

Invariants

If the following invariants are violated, the proxy will throw a TypeError:

• Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable data property.
• Cannot set the value of a property if the corresponding target object property is a non-configurable accessor property that has undefined as its [[Set]] attribute.
• In strict mode, a false return value from the set() handler will throw a TypeError exception.

Examples

Trap setting of a property value

The following code traps setting a property value.

const p = new Proxy({}, {
  set: function(target, prop, value, receiver) {
    target[prop] = value;
    console.log('property set: ' + prop + ' = ' + value);
    return true;
  }
})

console.log('a' in p);  // false

p.a = 10;               // "property set: a = 10"
console.log('a' in p);  // true
console.log(p.a);       // 10

Specifications

Browser compatibility

See also

• Proxy
• handler
• Reflect.set()