Parameters decorators -> Methods decorators -> Accessor/Property decorators -> Class decorators

> It is particularly important to note the [difference](https://stackoverflow.com/questions/48509376/whats-the-difference-between-reflect-getmetadata-and-reflect-getownmetadata) between getMetadata and getOwnMetadata.

There is a well defined order to how decorators applied to various declarations inside of a class are applied:

1.  *Parameter Decorators*, followed by *Method*, *Accessor*, or *Property Decorators* are applied for each instance member.
2.  *Parameter Decorators*, followed by *Method*, *Accessor*, or *Property Decorators* are applied for each static member.
3.  *Parameter Decorators* are applied for the constructor.
4.  *Class Decorators* are applied for the class.

```typescript

/**
 * Decorator Evaluation
 */
import { UserType } from './exp7-8';
import { UserService as ParentUserService } from './exp9-10';

const requiredMetadataKey = Symbol('required');

export const RequiredDecorator = (
    target: any,
    propertyKey: string | symbol,
    parameterIndex: number,
) => {
    const existingRequiredParameters: number[] =
        Reflect.getOwnMetadata(requiredMetadataKey, target, propertyKey) || [];
    existingRequiredParameters.push(parameterIndex);
    Reflect.defineMetadata(
        requiredMetadataKey,
        existingRequiredParameters,
        target,
        propertyKey,
    );
};

export const ValidateDecorator = (
    target: any,
    propertyName: string,
    descriptor: TypedPropertyDescriptor<(...args: any[]) => any>,
) => {
    const method = descriptor.value;
    descriptor.value = function (...args: any[]) {
        const requiredParameters: number[] = Reflect.getOwnMetadata(
            requiredMetadataKey,
            target,
            propertyName,
        );
        if (requiredParameters) {
            for (const parameterIndex of requiredParameters) {
                if (
                    parameterIndex >= args.length ||
                    args[parameterIndex] === undefined
                ) {
                    throw new Error('Missing required argument.');
                }
            }
        }

        return method.apply(this, args);
    };
};

class UserService extends ParentUserService {
    @ValidateDecorator
    createUser(@RequiredDecorator username?: string, id?: number) {
        const ids: number[] = this.users.map((userEntity) => userEntity.id);
        const newUser: UserType = {
            // If the ID parameter is not provided, the ID of the new user will be the maximum ID of all users plus 1.
            id: id || Math.max(...ids) + 1,
            // If the username parameter is not provided, a random string will be generated as the username.
            username: username || Math.random().toString(36).substring(2, 15),
        };
        this.users.push(newUser);
        return newUser;
    }
}

export const exp11 = () => {
    console.log();
    console.log(
        '-----------------------Decorator Evaluation-----------------------',
    );
    console.log(
        '-----------------------Provide mandatory validation for the username parameter.-----------------------',
    );
    console.log();
    const user = new UserService();
    user.createUser();
    console.log(user.getUsers());
    console.log();
    console.log(
        '-----------------------End-----------------------',
    );
};
```


10. Decorator Evaluation

Parameters decorators -> Methods decorators -> Accessor/Property decorators -> Class decorators

>...

Brian

Brian

10. Decorator Evaluation