class-properties

Decorate class properties in TypeScript to allow the class's property names to be known at runtime.

This allows objects to be checked for missing or excess properties compared to a class.

For more comprehensive decorator-based object validation, consider class-validator.

Installation

yarn add class-properties

Usage

Decorate class properties with the @Property decorator:

import { Property } from 'class-properties';

class Test {
  @Property()
  field1!: string;

  @Property()
  field2!: string;
}

Then retrieve the decorated property names as a string array at runtime:

import { getClassProperties } from 'class-properties';

const properties = getClassProperties(Test);
// properties = ['field1', 'field2']

You can also compare objects with the class's known properties to determine if it has extra or missing properties:

import { compareProperties } from 'class-properties';

const testObject = {
  field2: 'field2',
  field3: 'field3',
};

const result = compareProperties(testObject, Test);
// result.missing = ['field1']
// result.extra = ['field3']

Motivation

This may be useful for two reasons:

1. TypeScript's structural typing means that object literals may include excess properties and remain compatible with a type.
2. Since we are compiling to JavaScript there is not a runtime guarantee that the properties of an object match those which were expected.

We may need to confirm our assumptions of the properties that should be present if, for example, handling a data transfer object or passing a model to an ORM.

This is typically handled through a validation library, but in some scenarios a simpler approach may be sufficient.

Implementation

Property names are stored in the reflect-metadata store for the class's prototype under the key __class-properties.

FAQs

Is it possible to determine interface properties at runtime?

No. TypeScript interfaces do not exist in compiled JavaScript, and there is no way to decorate them.

Are property types or modifiers accessible?

No. Only property names are stored. TypeScript types do not exist in compiled JavaScript, so it is not possible to read them via decorator.