How to fix 'emitDecoratorMetadata' requires 'experimentalDecorators' to be enabled in TypeScript

What this error means

The "'emitDecoratorMetadata' requires 'experimentalDecorators' to be enabled" error appears during TypeScript compilation when your tsconfig.json has "emitDecoratorMetadata": true but lacks "experimentalDecorators": true. These two compiler options are interdependent. The emitDecoratorMetadata option enables TypeScript to emit runtime type metadata for decorated declarations when using the reflect-metadata library. This metadata includes design-time type information like parameter types, return types, and property types. However, this feature only makes sense when decorators are enabled, which is why experimentalDecorators must also be set to true. This error is commonly encountered when working with frameworks like NestJS, TypeORM, Angular, or any library that relies on dependency injection or runtime reflection. These frameworks use decorators extensively and need both the decorator syntax (experimentalDecorators) and type metadata (emitDecoratorMetadata) to function properly.

Typical symptoms

• Compilation fails with error mentioning 'emitDecoratorMetadata' and 'experimentalDecorators'
• Error occurs immediately when running tsc or starting the development server
• NestJS, TypeORM, or Angular projects fail to compile after configuration changes
• IDE shows configuration errors in tsconfig.json file
• Build fails in CI/CD pipeline after updating TypeScript configuration
• Error appears when trying to use @Injectable, @Entity, or other decorator-based features

Common causes

1. tsconfig.json has emitDecoratorMetadata enabled but experimentalDecorators is missing or false
2. Configuration file was manually edited and experimentalDecorators was accidentally removed
3. Copying tsconfig.json from one project to another with incomplete settings
4. Using a framework starter template that has incomplete TypeScript configuration
5. Extending a base tsconfig that doesn't include experimentalDecorators
6. Multiple tsconfig files in a monorepo with inconsistent decorator settings

How to fix "'emitDecoratorMetadata' requires 'experimentalDecorators' to be enabled"

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "target": "ES2020",
    "module": "commonjs",
    "lib": ["ES2020"],
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true
  }
}

# Check for compilation errors
npx tsc --noEmit

# Or run your build command
npm run build

# Start the development server
npm run start:dev

# Run TypeORM CLI commands
npx typeorm migration:run

# Kill any running ts-node processes
killall ts-node

# Restart your application
npx ts-node src/index.ts

# Find all tsconfig files
find . -name "tsconfig*.json" -not -path "*/node_modules/*"

my-monorepo/
├── tsconfig.base.json          # Base config
├── apps/
│   ├── api/
│   │   └── tsconfig.json       # Extends base
│   └── web/
│       └── tsconfig.json
└── packages/
    └── shared/
        └── tsconfig.json

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "strict": true
  }
}

{
  "extends": "../../tsconfig.base.json",
  "compilerOptions": {
    "outDir": "./dist",
    "rootDir": "./src"
  }
}

npm install reflect-metadata

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();

import "reflect-metadata";
import { createConnection } from "typeorm";
import { User } from "./entity/User";

createConnection().then(async connection => {
  // Your code here
});

Advanced notes

{
  "compilerOptions": {
    "experimentalDecorators": true,  // Use legacy decorators
    "emitDecoratorMetadata": true     // Emit type metadata
  }
}

import "reflect-metadata";

class UserService {
  constructor(private db: Database) {}
}

// TypeScript emits this metadata:
Reflect.getMetadata("design:paramtypes", UserService);
// Returns: [Database]

// Without metadata - manual type specification
@Column({ type: "varchar", length: 255 })
name: string;

// With metadata - automatic inference
@Column()
name: string;  // TypeORM knows this is varchar

# Show effective configuration
npx tsc --showConfig

# Check which files are included
npx tsc --listFiles | head -20

Frequently asked questions

What does "'emitDecoratorMetadata' requires 'experimentalDecorators' to be enable" mean?

json has "emitDecoratorMetadata": true but lacks "experimentalDecorators": true. These two compiler options are interdependent. The emitDecoratorMetadata option enables TypeScript to emit runtime type metadata for decorated declarations when using the reflect-metadata library.

How do I fix "'emitDecoratorMetadata' requires 'experimentalDecorators' to be enable"?

This TypeScript configuration error occurs when you enable emitDecoratorMetadata in tsconfig.json without also enabling experimentalDecorators. Both options must be enabled together because decorator metadata depends on the experimental decorator feature.

What causes "'emitDecoratorMetadata' requires 'experimentalDecorators' to be enable"?

tsconfig.json has emitDecoratorMetadata enabled but experimentalDecorators is missing or false Configuration file was manually edited and experimentalDecorators was accidentally removed Copying tsconfig.json from one project to another with incomplete settings

Is the "'emitDecoratorMetadata' requires 'experimentalDecorators' to be enable" error serious?

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

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Official resources & further reading

• 📚TypeScript Official Documentation
• 📖
  TypeScript TSConfig - emitDecoratorMetadata(Official Docs)
• 📖
  TypeScript Handbook - Decorators(Official Docs)
• Medium - Solving Experimental Decorators Warning
• 🐙
  Deno GitHub - experimentalDecorators & emitDecoratorMetadata Discussion(GitHub)
• BobbyHadz - Experimental Support for Decorators Warning
• 🐙
  Nx - Enable emitDecoratorMetadata in tsconfig(GitHub)
• TechSparx - Reflection and Reflection Metadata APIs
• npm - reflect-metadata
• GitHub - Reflect Metadata Prototype