Registration metadata
Record property names for a registration or metadata system during class evaluation.
TypeScript · Decorator
PropertyDecorator
A legacy property decorator is a runtime callback attached immediately before a class property. It receives the class constructor or prototype and the property name, so it can observe the declaration but cannot directly modify its descriptor.
How it works
A legacy property decorator is placed immediately before a class property. At runtime, TypeScript calls it with the class constructor for a static property or the class prototype for an instance property, plus the property name. It can observe the declaration, but it cannot directly replace or redefine the property because no descriptor is provided.
Syntax
function propertyDecorator(target: Function | Object, propertyKey: string | symbol): voidParameters
| Name | Type | Description |
|---|---|---|
target | Function | Object | The class constructor for a static property, or the class prototype for an instance property. |
propertyKey | string | symbol | The declared property name. |
Returns
The decorator callback returns void; the documented legacy property-decorator protocol does not provide a property descriptor to replace.
Key points
- Write the decorator directly before the property it decorates.
- The decorator expression runs at runtime when the decorated class is evaluated.
- The legacy callback receives two arguments: the target object and the property name.
- Static properties use the class constructor as the target; instance properties use the prototype.
- The second callback argument identifies the decorated property by name.
- This legacy decorator is observational rather than a mechanism for replacing the property.
Common mistakes
- Property decorators are unavailable in declaration files and other ambient contexts, including declare classes.
- This decorator form observes a named property declaration; it cannot directly alter the property's descriptor or implementation.
- The first argument differs for static and instance properties: it is the constructor for static members and the prototype for instance members.
Example: Observing a property declaration
function logProperty(target: object, propertyKey: string | symbol): void {
console.log(String(propertyKey));
}
class User {
@logProperty
name = "Ada";
}
new User();The decorator observes the declared property name when the class is evaluated.
When to use it
Declaration observation
Inspect whether specific instance or static properties are declared.
Framework integration
Build lightweight framework conventions around decorated class members.
Related
DecoratorMethodDecoratorParameterDecoratorexperimentalDecoratorsClass Decorators