TSConfig

emitDecoratorMetadata

Enables experimental support for emitting type metadata for decorators which works with the module reflect-metadata.

For example, here is the TypeScript

ts
function LogMethod(target: any, propertyKey: string | symbol, descriptor: PropertyDescriptor) {
console.log(target);
console.log(propertyKey);
console.log(descriptor);
}
 
class Demo {
@LogMethod
public foo(bar: number) {
// do nothing
}
}
 
const demo = new Demo();
Try

With emitDecoratorMetadata not set to true (default) the emitted JavaScript is:

ts
"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
return c > 3 && r && Object.defineProperty(target, key, r), r;
};
function LogMethod(target, propertyKey, descriptor) {
console.log(target);
console.log(propertyKey);
console.log(descriptor);
}
class Demo {
foo(bar) {
// do nothing
}
}
__decorate([
LogMethod
], Demo.prototype, "foo", null);
const demo = new Demo();
 
Try

With emitDecoratorMetadata set to true the emitted JavaScript is:

ts
"use strict";
var __decorate = (this && this.__decorate) || function (decorators, target, key, desc) {
var c = arguments.length, r = c < 3 ? target : desc === null ? desc = Object.getOwnPropertyDescriptor(target, key) : desc, d;
if (typeof Reflect === "object" && typeof Reflect.decorate === "function") r = Reflect.decorate(decorators, target, key, desc);
else for (var i = decorators.length - 1; i >= 0; i--) if (d = decorators[i]) r = (c < 3 ? d(r) : c > 3 ? d(target, key, r) : d(target, key)) || r;
return c > 3 && r && Object.defineProperty(target, key, r), r;
};
var __metadata = (this && this.__metadata) || function (k, v) {
if (typeof Reflect === "object" && typeof Reflect.metadata === "function") return Reflect.metadata(k, v);
};
function LogMethod(target, propertyKey, descriptor) {
console.log(target);
console.log(propertyKey);
console.log(descriptor);
}
class Demo {
foo(bar) {
// do nothing
}
}
__decorate([
LogMethod,
__metadata("design:type", Function),
__metadata("design:paramtypes", [Number]),
__metadata("design:returntype", void 0)
], Demo.prototype, "foo", null);
const demo = new Demo();
 
Try