How to fix Experimental support for decorators is a feature that is subject to change in a future release. Set the 'experimentalDecorators' option in your 'tsconfig' to remove this warning. in TypeScript

### Legacy vs Standard Decorators

TypeScript 5.0+ supports the official ECMAScript decorators standard, which is different from legacy experimental decorators:

Legacy Decorators (experimentalDecorators):
- Based on the original TC39 Stage 2 proposal (2014-2022)
- Used by Angular, NestJS, TypeORM, MobX, InversifyJS
- Requires experimentalDecorators: true
- Decorator functions receive different parameters than standard decorators

Standard Decorators (TypeScript 5.0+):
- Based on the finalized TC39 Stage 3 decorators proposal
- Enabled by default in TypeScript 5.0+
- Different decorator function signature
- Not compatible with libraries built for legacy decorators

Important: You cannot mix both types. If your framework uses legacy decorators, you must enable experimentalDecorators. Most frameworks haven't migrated to standard decorators yet.

### emitDecoratorMetadata Option

The emitDecoratorMetadata option works alongside experimentalDecorators and emits runtime type information:

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

When enabled, TypeScript emits type metadata that can be accessed at runtime via the reflect-metadata library:

import "reflect-metadata";

class MyClass {
  @LogType
  myMethod(param: string): number {
    return 42;
  }
}

// At runtime, you can read the parameter types:
const paramTypes = Reflect.getMetadata("design:paramtypes", MyClass.prototype, "myMethod");
console.log(paramTypes); // [String]

const returnType = Reflect.getMetadata("design:returntype", MyClass.prototype, "myMethod");
console.log(returnType); // Number

This is essential for:
- Dependency Injection frameworks (Angular, NestJS, InversifyJS)
- Validation libraries (class-validator, class-transformer)
- ORMs (TypeORM) that need runtime type information

### Common Framework Requirements

Angular:

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

NestJS:

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

TypeORM:

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

MobX:

{
  "compilerOptions": {
    "experimentalDecorators": true
  }
}

### Decorator Syntax Examples

Class decorators:

@Component({
  selector: 'app-root'
})
class AppComponent {}

Method decorators:

class UserService {
  @Get('/users')
  findAll() {}
}

Property decorators:

class User {
  @Column()
  name: string;
}

Parameter decorators:

class UserController {
  create(@Body() userData: CreateUserDto) {}
}

### Migration Considerations

If you're starting a new project, consider:
1. Stick with legacy decorators if using Angular, NestJS, TypeORM - they haven't migrated yet
2. Use standard decorators for greenfield projects without framework dependencies
3. Wait for framework updates before migrating existing projects

Watch for TypeScript 6.0 which may change decorator defaults or deprecate legacy decorators entirely.

### Troubleshooting

If enabling experimentalDecorators doesn't work:

1. Check TypeScript version: Decorators require TypeScript 1.5+

npx tsc --version

2. Verify node_modules TypeScript: Your IDE might use a different TypeScript version

ls -la node_modules/typescript

3. Clear build cache:

rm -rf dist/
rm -rf .tsbuildinfo

4. Check .vscode/settings.json: Local workspace settings can override tsconfig

{
  "typescript.tsdk": "node_modules/typescript/lib"
}