typescript decorator implemented on a class that is nested within another class

Is it possible to decorate a nested property within a class? Let's explore with an example:

    function log(name: string = 'DecoratedProp') {
        return function logHandler(target: any, field: any) {
            // get the key
            Object.defineProperty(target, field, {
                get() {
                    console.info('Accessing:' + name, field)
                    return this[name];
                },
                set(value: any): void {
                    console.warn('Setting:' + name, field)
                    this[name] = value;
                }
            })
        };
    }
    class Nested {
        @log('Nested') data: string;
    }
    export class DecoratedModel {
        nested: Nested = {
            data: 'something',
            // decorating here won't work, e.g.
            // @log() somethingElse; -> breaks
        };
        @log() topLevel: string = '33';
    }
    const deco = new DecoratedModel();
    console.log(deco.topLevel);         // logs stuff
    console.log(deco.nested.data);      // doesn't log

Why isn't my nested property showing as decorated?

Answer №1

When you assign a static object that matches the class interface, the compiler is satisfied. However, since it does not have the decorated prototype, it is not actually an instance of the class you have decorated.

To resolve this issue, you will need to instantiate the Nested class with new, and then it will work correctly...

class Nested {
  @log('Nested') data: string;

  constructor(data: string) {
    this.data = data
  }
}

export class DecoratedModel {
  nested: Nested = new Nested('something');

  @log('toplevel') topLevel: string = '33';
}

const deco = new DecoratedModel();
console.log(deco2.topLevel); // logs
console.log(deco2.nested.data); // also logs!

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