project-property.decorator.ts

interface Property {
  hasValue?: boolean;
  lastInputPropertyValue?: any;
  currentValue?: any;
}

export function ProjectProperty<T, K extends keyof T>(inputPropertyName: K, projectFn: (input: T[K]) => any) {
  const propertySymbol = Symbol();

  return (target: T, propertyKey: PropertyKey) => {
    Object.defineProperty(target, propertyKey, { get: getValue });
  };

  function getProperty(instance: { [propertySymbol]: Property }) {
    return instance[propertySymbol] || (instance[propertySymbol] = {});
  }

  function getValue(this: any) {
    const property = getProperty(this);
    const inputPropertyValue = this[inputPropertyName];

    if (!property.hasValue || inputPropertyValue !== property.lastInputPropertyValue) {
      property.hasValue = true;
      property.lastInputPropertyValue = inputPropertyValue;
      property.currentValue = projectFn(inputPropertyValue);
    }

    return property.currentValue;
  }
}