How to fix Decorators are not valid on parameters unless 'emitDecoratorMetadata' is enabled in TypeScript

What this error means

Parameter decorators are a TypeScript feature that allows you to attach metadata to function or method parameters. They are commonly used in dependency injection frameworks like NestJS, Angular, and InversifyJS to automatically resolve dependencies at runtime. When TypeScript compiles code with parameter decorators, it needs to emit type metadata that describes the parameter types. This metadata is stored using the reflect-metadata library and accessed at runtime. Without the emitDecoratorMetadata option enabled, TypeScript cannot emit this critical type information, causing the compiler to reject parameter decorators entirely. This error is distinct from general decorator errors because parameter decorators specifically require metadata reflection. While class, method, and property decorators can work without emitDecoratorMetadata, parameter decorators cannot function without knowing the runtime types of parameters—information only available through metadata emission. The error commonly appears in dependency injection scenarios where frameworks need to know what types to inject into constructor or method parameters.

Typical symptoms

• TypeScript compiler error when using @Inject(), @Param(), or custom parameter decorators
• Error specifically mentions parameter decorators and emitDecoratorMetadata
• Code works in JavaScript but fails TypeScript compilation
• Dependency injection frameworks like NestJS fail to compile
• Parameter decorators on constructor parameters trigger compilation errors
• IDE (VS Code, WebStorm) shows red underlines on parameter decorators

Common causes

1. emitDecoratorMetadata is not set to true in tsconfig.json
2. experimentalDecorators is disabled (both flags required for parameter decorators)
3. Using TypeScript 5.0+ standard decorators which don't support parameter decorators
4. tsconfig.json is not being read correctly (wrong directory or extends chain broken)
5. reflect-metadata package not installed or imported in entry file
6. Multiple tsconfig files with conflicting settings in a monorepo
7. Attempting to use parameter decorators with isolatedModules: true (limited support)

How to fix "Decorators are not valid on parameters unless 'emitDecoratorMetadata' is enabled"

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "target": "ES2020",
    "module": "commonjs"
  }
}

npm install reflect-metadata
# or
yarn add reflect-metadata
# or
pnpm add reflect-metadata

// main.ts - FIRST import, before anything else
import "reflect-metadata";

import { NestFactory } from "@nestjs/core";
import { AppModule } from "./app.module";

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
bootstrap();

# Check which config file TypeScript is using
npx tsc --showConfig

{
  "extends": "./tsconfig.base.json",  // Must exist
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

# Compile with specific config
npx tsc -p packages/api/tsconfig.json

{
  "compilerOptions": {
    "experimentalDecorators": true,  // REQUIRED for parameter decorators
    "emitDecoratorMetadata": true,
    "target": "ES2022"
  }
}

// tsconfig.json
{
  "ts-node": {
    "compilerOptions": {
      "experimentalDecorators": true,
      "emitDecoratorMetadata": true
    }
  }
}

ts-node --experimental-decorators --emit-decorator-metadata src/main.ts

// webpack.config.js
module.exports = {
  module: {
    rules: [
      {
        test: /\.tsx?$/,
        loader: "ts-loader",
        options: {
          compilerOptions: {
            experimentalDecorators: true,
            emitDecoratorMetadata: true,
          },
        },
      },
    ],
  },
};

{
  "compilerOptions": {
    "isolatedModules": true,  // Can cause issues
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

// tsconfig.build.json (for esbuild/swc)
{
  "extends": "./tsconfig.json",
  "compilerOptions": {
    "isolatedModules": false  // Only for type checking
  }
}

// .swcrc
{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "decorators": true
    },
    "transform": {
      "legacyDecorator": true,
      "decoratorMetadata": true
    }
  }
}

Advanced notes

class UserService {
  constructor(@Inject() private db: DatabaseService) {}
}

// TypeScript emits (conceptually):
// design:type - the type of the decorated element
// design:paramtypes - parameter types (for constructors/methods)
// design:returntype - return type (for methods)

__metadata("design:paramtypes", [DatabaseService])

import "reflect-metadata";
import { Injectable, Controller, Get, Param } from "@nestjs/common";

@Injectable()
export class UsersService {
  findOne(id: string) {
    return { id, name: "John" };
  }
}

@Controller("users")
export class UsersController {
  // Parameter decorator @Param requires emitDecoratorMetadata
  constructor(private usersService: UsersService) {}

  @Get(":id")
  findOne(@Param("id") id: string) {
    return this.usersService.findOne(id);
  }
}

monorepo/
├── packages/
│   ├── api/
│   │   └── tsconfig.json  ← Must have flags
│   ├── shared/
│   │   └── tsconfig.json  ← Must have flags
│   └── web/
│       └── tsconfig.json  ← May not need them
└── tsconfig.base.json     ← Can set here too

// tsconfig.base.json
{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

// packages/api/tsconfig.json
{
  "extends": "../../tsconfig.base.json"
}

npx tsc --outFile output.js src/main.ts
cat output.js | grep "design:paramtypes"

__metadata("design:paramtypes", [DatabaseService, LoggerService])

Frequently asked questions

What does "Decorators are not valid on parameters unless 'emitDecoratorMetadata'" mean?

Parameter decorators are a TypeScript feature that allows you to attach metadata to function or method parameters. They are commonly used in dependency injection frameworks like NestJS, Angular, and InversifyJS to automatically resolve dependencies at runtime. When TypeScript compiles code with parameter decorators, it needs to emit type metadata that describes the parameter types.

How do I fix "Decorators are not valid on parameters unless 'emitDecoratorMetadata'"?

This TypeScript error occurs when you use parameter decorators without enabling the emitDecoratorMetadata compiler option. Parameter decorators rely on metadata reflection to function properly, which requires both experimentalDecorators and emitDecoratorMetadata to be enabled in tsconfig.json.

What causes "Decorators are not valid on parameters unless 'emitDecoratorMetadata'"?

emitDecoratorMetadata is not set to true in tsconfig.json experimentalDecorators is disabled (both flags required for parameter decorators) Using TypeScript 5.0+ standard decorators which don't support parameter decorators

Is the "Decorators are not valid on parameters unless 'emitDecoratorMetadata'" error serious?

It is a medium-severity error — more of a blocker than a risk. The steps above resolve it.

Related errors

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• Cannot find name 'process'. Do you need to install type definitions for node? Try `npm i --save-dev @types/node` and then add 'node' to the types field in your tsconfig.

  How to fix "Cannot find name 'process'" in TypeScript

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Official resources & further reading

• 📚TypeScript Official Documentation
• 📖
  TypeScript TSConfig - emitDecoratorMetadata(Official Docs)
• 📖
  TypeScript Handbook - Decorators(Official Docs)
• 📖
  TypeScript 5.0 Release Notes(Official Docs)
• LogRocket - Practical Guide to TypeScript Decorators
• Trilon - NestJS Metadata Deep Dive
• DEV.to - Passing Metadata with Decorators for DI
• Medium - Why Use Injectable Decorator with NestJS
• 🐙
  TypeScript GitHub - emitDecoratorMetadata Issues(GitHub)