npm install reflect-metadata
Declarative definition of metadata:
class C {
@Reflect.metadata(metadataKey, metadataValue)
method() {
}
}
Imperative definition of metadata:
Reflect.defineMetadata(metadataKey, metadataValue, C.prototype, "method");
Imperative introspection of metadata:
let obj = new C();
let metadataValue = Reflect.getMetadata(metadataKey, obj, "method");
// define metadata on an object or property
Reflect.defineMetadata(metadataKey, metadataValue, target);
Reflect.defineMetadata(metadataKey, metadataValue, target, propertyKey);
// check for presence of a metadata key on the prototype chain of an object or property
let result = Reflect.hasMetadata(metadataKey, target);
let result = Reflect.hasMetadata(metadataKey, target, propertyKey);
// check for presence of an own metadata key of an object or property
let result = Reflect.hasOwnMetadata(metadataKey, target);
let result = Reflect.hasOwnMetadata(metadataKey, target, propertyKey);
// get metadata value of a metadata key on the prototype chain of an object or property
let result = Reflect.getMetadata(metadataKey, target);
let result = Reflect.getMetadata(metadataKey, target, propertyKey);
// get metadata value of an own metadata key of an object or property
let result = Reflect.getOwnMetadata(metadataKey, target);
let result = Reflect.getOwnMetadata(metadataKey, target, propertyKey);
// get all metadata keys on the prototype chain of an object or property
let result = Reflect.getMetadataKeys(target);
let result = Reflect.getMetadataKeys(target, propertyKey);
// get all own metadata keys of an object or property
let result = Reflect.getOwnMetadataKeys(target);
let result = Reflect.getOwnMetadataKeys(target, propertyKey);
// delete metadata from an object or property
let result = Reflect.deleteMetadata(metadataKey, target);
let result = Reflect.deleteMetadata(metadataKey, target, propertyKey);
// apply metadata via a decorator to a constructor
@Reflect.metadata(metadataKey, metadataValue)
class C {
// apply metadata via a decorator to a method (property)
@Reflect.metadata(metadataKey, metadataValue)
method() {
}
}
Use properties rather than a separate API.
Obvious downside is that this can be a lot of code:
function ParamTypes(...types) {
return (target, propertyKey) => {
const symParamTypes = Symbol.for("design:paramtypes");
if (propertyKey === undefined) {
target[symParamTypes] = types;
}
else {
const symProperties = Symbol.for("design:properties");
let properties, property;
if (Object.prototype.hasOwnProperty.call(target, symProperties)) {
properties = target[symProperties];
}
else {
properties = target[symProperties] = {};
}
if (Object.prototype.hasOwnProperty.call(properties, propertyKey)) {
property = properties[propertyKey];
}
else {
property = properties[propertyKey] = {};
}
property[symParamTypes] = types;
}
};
}
Though it may seem counterintuitive, the methods on Reflect place the parameters for the metadata key and metadata value before the target or property key. This is due to the fact that the property key is the only optional parameter in the argument list. This also makes the methods easier to curry with Function#bind. This also helps reduce the overall footprint and complexity of a metadata-producing decorator that could target both a class or a property:
function ParamTypes(...types) {
// as propertyKey is effectively optional, its easier to use here
return (target, propertyKey) => { Reflect.defineMetadata("design:paramtypes", types, target, propertyKey); }
// vs. having multiple overloads with the target and key in the front:
//
// return (target, propertyKey) => {
// if (propertyKey === undefined) {
// Reflect.defineMetadata(target, "design:paramtypes", types);
// }
// else {
// Reflect.defineMetadata(target, propertyKey, "design:paramtypes", types);
// }
// }
//
// vs. having a different methods for the class or a property:
//
// return (target, propertyKey) => {
// if (propertyKey === undefined) {
// Reflect.defineMetadata(target, "design:paramtypes", types);
// }
// else {
// Reflect.definePropertyMetadata(target, propertyKey, "design:paramtypes", types);
// }
// }
}
To enable experimental support for metadata decorators in your TypeScript project, you must add "experimentalDecorators": true
to your tsconfig.json file.
To enable experimental support for auto-generated type metadata in your TypeScript project, you must add "emitDecoratorMetadata": true
to your tsconfig.json file.