How to combine multiple property decorators in Typescript?

Question

I have a class Template with a property _id which has decorators from class-transformer and typed-graphql

Answer 1

import { Expose, Type, TypeOptions, ExposeOptions } from 'class-transformer';

/**
 * Combines @Expose then @Types decorators.
 * @param exposeOptions options that passes to @Expose()
 * @param typeFunction options that passes to @Type()
 */
function ExposeToGraphQL(exposeOptions?: ExposeOptions, typeFunction?: (type?: TypeOptions) => Function) {
  const exposeFn = Expose(exposeOptions);
  const typeFn = Type(typeFunction);

  return function (target: any, key: string) {
    typeFn(target, key);
    exposeFn(target, key);
  }
}

Then you can use that decorator as follow:

class Template extends Typegoose {
    @ExposeToGraphQL(/*exposeOptions*/ undefined, /*typeFunction*/ () => String)
    @Field(() => ID)
    public _id?: mongoose.Types.ObjectId;
}

---

You can find official documentation for decorator in this link.

@Expose and @Type() are basically Decorator Factories. The main purpose of a decorator factory:

• it return a function
• that function will be called at runtime (right after the class, in this case is Template, was defined) with 2 arguments:
  • class prototype (Template.prototype)
  • name of the property which the decorator attached to (_id).

If two or more decorators are attached to a same property (called as Decorator Composition), they are evaluated as follow:

• The factory functions are executed in the same order as they are written in code
• The functions returned by factory functions are executed in reversed order