検証環境

バージョン

library	version
Node.js	v16.13.1
Typescript	v4.5.4
reflect-metadata	v0.1.13

tsconfig.json

必要な設定を有効化しておく。

{
  "experimentalDecorators": true,
  "emitDecoratorMetadata": true
}

検証方法

jestテストファイルに型チェックできるProperty Decoratorを用意した。
別にDecoratorにする必要はないがプロパティ定義と得られる型をまとめて記述できて都合がよかった。

function CheckType(expectedType: any): PropertyDecorator {
  return function (target: Object, propertyKey: string | symbol) {
    const reflectedType: Function = Reflect.getMetadata(
      'design:type',
      target,
      propertyKey
    );

    expect(reflectedType).toBe(expectedType);
  };
}

class Test {
  @CheckType(String) s: string;
}

結果

プロパティ定義	expectedType	備考
prop: string	String
prop: number	Number
prop: Boolean	Boolean
prop: true	Boolean
prop: Date	Date
prop: Symbol	Object
prop: Map	Map	ジェネリック型の内容まではわからない
prop: Set	Set
prop: SomeClass	SomeClass
prop: SomeInterface	Object	`interface SomeInterface {}`
prop: ObjectType	Object	`type ObjectType = {};`
prop: StringType	String	`type StringType = '';`
prop: Function	Function
prop: () => string	Function
prop: string[]	Array
prop: [string, number]	Array
prop: {}	Object
prop: Object	Object
prop: NumberEnum	Number	`enum NumberEnum { A = 0, B }`
prop: StringEnum	String	`enum StringEnum { A = 'a', B = 'b' }`
prop: MixEnum	Object	`enum MixEnum { A = 'a', B = 1 }` : union型と同様？
prop	Object	型定義なし
prop = ''	Object	初期化していても型定義が無ければObjectになる
prop: null	undefined
prop: undefined	undefined
prop: any	Object
prop: unknown	Object
prop: void	undefined
prop: never	undefined
prop: string \| number	Object
prop: 'a' \| 'b'	String	union型
prop: string \| null	String
prop: string \| undefined	String
prop: string \| any	Object
prop: string \| unknown	Object
prop: string \| void	Object
prop: string \| string[]	Object
prop: string & number	Object
prop: 'a' & 'b'	String	intersection型。調べた範囲ではunionと同じ結果
prop: string & null	String
prop: string & undefined	String
prop: string & any	Object
prop: string & unknown	Object
prop: string & void	Object
prop: string & string[]	Object
prop: T	Object	`class Test<T>`
prop: T2	String	`class Test<T extends string>`

検証コード全体

import 'reflect-metadata';

function CheckType(expectedType: any): PropertyDecorator {
  return function (target: Object, propertyKey: string | symbol) {
    const reflectedType: Function = Reflect.getMetadata(
      'design:type',
      target,
      propertyKey
    );

    expect(reflectedType).toBe(expectedType);
  };
}

describe('Reflect.getMetadata("design:type")', () => {
  test('', () => {
    class SomeClass {}
    interface MyInterface {}
    type ObjectType = {};
    type StringType = '';
    type UnionType1 = string | number;
    type UnionType2 = 'a' | 'b';
    type IntersectionType = string & number;

    enum NumberEnum {
      A = 0,
      B,
    }

    enum StringEnum {
      A = 'a',
      B = 'b',
    }

    enum MixEnum {
      A = 'a',
      B = 1,
    }

    class Test<T, T2 extends string> {
      @CheckType(String) prop0: string;
      @CheckType(String) prop1: string = '';
      @CheckType(Number) prop2: number;
      @CheckType(Boolean) prop3: Boolean;
      @CheckType(Boolean) prop4: true;
      @CheckType(Date) prop5: Date;
      @CheckType(Object) prop6: Symbol;
      @CheckType(Map) prop7: Map<number, string>;
      @CheckType(Set) prop8: Set<string>;
      @CheckType(SomeClass) prop9: SomeClass;
      @CheckType(Object) prop10: MyInterface;
      @CheckType(Object) prop11: ObjectType;
      @CheckType(String) prop12: StringType;
      @CheckType(Function) prop13: Function;
      @CheckType(Function) prop14: () => string;
      @CheckType(Array) prop15: string[];
      @CheckType(Object) prop16: {};
      @CheckType(Object) prop17: Object;
      @CheckType(Object) prop18: unknown;
      @CheckType(Object) prop19: any;
      @CheckType(Number) prop20: NumberEnum;
      @CheckType(String) prop21: StringEnum;
      @CheckType(Object) prop22: MixEnum;
      @CheckType(Object) prop23: string | number;
      @CheckType(String) prop24: 'a' | 'b';
      @CheckType(String) prop25: string | undefined;
      @CheckType(Object) prop26: string | void;
      @CheckType(String) prop27: string | null;
      @CheckType(Object) prop28: string | any;
      @CheckType(Object) prop29: string | unknown;
      @CheckType(Object) prop30: string | string[];
      @CheckType(Object) prop31: string & number;
      @CheckType(String) prop32: 'a' & 'b';
      @CheckType(String) prop33: string & undefined;
      @CheckType(Object) prop34: string & void;
      @CheckType(String) prop35: string & null;
      @CheckType(Object) prop36: string & any;
      @CheckType(Object) prop37: string & unknown;
      @CheckType(Object) prop38: string & string[];
      @CheckType(Array) prop39: [string, number];
      @CheckType(undefined) prop40: void;
      @CheckType(undefined) prop41: never;
      @CheckType(Object) prop42;
      @CheckType(Object) prop43 = '';
      @CheckType(undefined) prop44: null;
      @CheckType(undefined) prop45: undefined;
      @CheckType(Object) prop46: T;
      @CheckType(String) prop47: T2;
    }
  });
});