handler.set()
The handler.set()
method is a trap for the [[Set]]
object internal method, which is used by operations such as using property accessors to set a property's value.
Try it
Syntax
js
new Proxy(target, {
set(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"
, andobj
is not a proxy, and has no own property.name
, but it has a proxy on its prototype chain. That proxy'sset()
handler will be called, andobj
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 returnsfalse
, and the assignment happened in strict-mode code, aTypeError
will be thrown.
Description
Interceptions
This trap can intercept these operations:
- Property assignment:
proxy[foo] = bar
andproxy.foo = bar
Reflect.set()
Or any other operation that invokes the [[Set]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
- 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 theset()
handler will throw aTypeError
exception.
Examples
Trap setting of a property value
The following code traps setting a property value.
js
const p = new Proxy(
{},
{
set(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
Specification |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-set-p-v-receiver |
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