reflectable.ts

import { strict as assert } from 'assert';

/**
 * This class supports retrieving a class type name at run-time. It requires TypeScript decorators.
 * Usage: Register the typename with `@TypeName.decorate(...)`
 * 
 * @TypeName.decorate("Foo")
 * class Foo {
 *     readonly text: string = "text";
 * }
 * 
 * const typeName = TypeName.tryGet(new Foo());
 * assert(typeName === "Foo")
 * 
 * class Bar {}
 * const typeName = TypeName.tryGet(new Bar());
 * assert(typeName === undefined)
 * 
 */

class TypeName {
	private constructor() {}

	private static readonly typeNameSymbol = Symbol("typeName");

	public static decorate(name: string): Function {
		return function getExtendedContructor<Base extends { new(...args: any[]): any }>(target: Base) {
			return class Constructor extends target {
				constructor(...args: any[]) {
					super(...args);
					(this as any)[TypeName.typeNameSymbol] = name;
				}
			}
		}
	}

	public static tryGet(o: any): string | undefined {
		if (o === undefined || o === null || typeof o[TypeName.typeNameSymbol] !== typeof "")
			return undefined;
		return o[TypeName.typeNameSymbol];
	}
}

function arraysEqual(array1: Array<unknown>, array2: Array<unknown>) {
	return array1 && array2 && array1.length === array2.length && array1.every((v, i) => v === array2[i]);
}

@TypeName.decorate("Foo")
class Foo {
	readonly text: string = "text";
	public static readonly staticText = "staticText";
}
const foo = new Foo();
assert(TypeName.tryGet(foo) === "Foo");
assert(arraysEqual(Object.getOwnPropertyNames(foo), ["text"]));
assert(arraysEqual(Object.keys(foo), ["text"]));
assert(arraysEqual(Object.getOwnPropertySymbols(foo), [(<any>TypeName).typeNameSymbol]));
assert(typeof Foo.staticText === typeof "");

class Bar {
	readonly count: number = 0;
	public static readonly staticCount = 0;
}
const bar = new Bar();
assert(TypeName.tryGet(bar) === undefined);
assert(arraysEqual(Object.getOwnPropertyNames(bar), ["count"]));
assert(arraysEqual(Object.keys(bar), ["count"]));
assert(arraysEqual(Object.getOwnPropertySymbols(bar), []));
assert(typeof Bar.staticCount === typeof 0);