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reflect-metadata
Advanced tools
The reflect-metadata npm package is a library that allows developers to perform metadata reflection operations in JavaScript. It is based on the metadata reflection API that is proposed for ECMAScript and is used by frameworks like Angular and decorators to add and read metadata annotations.
Metadata Reflection
Allows defining metadata on a class or property. The code sample shows how to define custom metadata on a class 'MyClass' with a key 'custom' and a value 'metadataValue'.
Reflect.defineMetadata('custom', 'metadataValue', MyClass);
Metadata Retrieval
Enables retrieval of metadata from a class or property. The code sample demonstrates how to retrieve metadata from a class 'MyClass' using the key 'custom'.
let metadataValue = Reflect.getMetadata('custom', MyClass);
Metadata Deletion
Provides functionality to delete metadata from a class or property. The code sample illustrates how to delete metadata associated with a class 'MyClass' using the key 'custom'.
Reflect.deleteMetadata('custom', MyClass);
core-decorators is a library of decorators for JavaScript/TypeScript that provides similar functionality to reflect-metadata. It offers decorators for various purposes such as memoization, autobinding, and deprecation, but does not provide the same low-level metadata reflection API.
class-transformer is a package that allows developers to transform plain objects to class instances and vice versa. It uses decorators to annotate classes and properties, similar to how reflect-metadata uses metadata, but it focuses more on the transformation aspect rather than the reflection of metadata.
class-validator is a package that uses decorators to perform validation on class properties. It leverages metadata to store validation rules, which is a concept similar to reflect-metadata. However, its primary focus is on validation rather than general metadata reflection.
NOTE: Now that both Decorators and
Decorator Metadata have achieved Stage 3 within TC39, the API
proposed below is no longer being considered for standardization. However, this package will continue to support
projects that leverage TypeScript's legacy --experimentalDecorators
option as some projects may not be able to migrate
to use standard decorators.
npm install reflect-metadata
// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Supports ESM and CommonJS.
// - Contains internal polyfills for `Map`, `Set`, and `WeakMap` for older runtimes.
import "reflect-metadata";
// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Supports ESM and CommonJS.
// - Requires runtime support for `"exports"` in `package.json`.
// - Does not include internal polyfills.
import "reflect-metadata/lite";
// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Contains internal polyfills for `Map`, `Set`, and `WeakMap` for older runtimes.
require("reflect-metadata");
// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Requires runtime support for `"exports"` in `package.json`.
// - Does not include internal polyfills.
require("reflect-metadata/lite");
<script>
HTML
<!-- Modifies global `Reflect` object (or defines one in ES5 runtimes). -->
<!-- Contains internal polyfills for `Map`, `Set`, and `WeakMap` for older runtimes. -->
<script src="path/to/reflect-metadata/Reflect.js"></script>
<!-- Modifies global `Reflect` object (or defines one in ES5 runtimes). -->
<!-- Does not include internal polyfills. -->
<script src="path/to/reflect-metadata/ReflectLite.js"></script>
Script
// - Makes types available in your editor.
/// <reference path="path/to/reflect-metadata/standalone.d.ts" />
class C {
@Reflect.metadata(metadataKey, metadataValue)
method() {
}
}
Reflect.defineMetadata(metadataKey, metadataValue, C.prototype, "method");
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() {
}
}
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;
}
};
}
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);
// }
// }
}
"experimentalDecorators": true
to your tsconfig.json file."emitDecoratorMetadata": true
to your tsconfig.json file.
FAQs
Polyfill for Metadata Reflection API
The npm package reflect-metadata receives a total of 10,345,006 weekly downloads. As such, reflect-metadata popularity was classified as popular.
We found that reflect-metadata demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 1 open source maintainer collaborating on the project.
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