Implementing GetPropertyNames in Typescript

I am currently working on a Linq-like library for Typescript and wanted to implement something like GetProperties of C# in Typescript / Javascript. The more I work with Typescript and generics, the clearer picture I get of that you usually have to have an instantiated object with intialized properties to get any useful information out at runtime about properties of a class. But it would be nice to retrieve information anyways just from the constructor function object, or an array of objects and be flexible about this. I was following a question thread on Stack Overflow and found a good answer that helped me out: https://stackoverflow.com/questions/40636292/get-properties-of-a-class/59586570#59586570 Here is what I ended up with for now. First off, I define Array prototype method ('extension method' for you C# developers).

export { } //creating a module of below code
declare global {
  interface Array>T< {
    GetProperties>T<(TClass: Function, sortProps: boolean): string[];
} }
The GetProperties method then looks like this, inspired by madreasons answer.

if (!Array.prototype.GetProperties) {
  Array.prototype.GetProperties = function >T<(TClass: any = null, sortProps: boolean = false): string[] {
    if (TClass === null || TClass === undefined) {
      if (this === null || this === undefined || this.length === 0) {
        return []; //not possible to find out more information - return empty array
      }
    }
    // debugger
    if (TClass !== null && TClass !== undefined) {
      if (this !== null && this !== undefined) {
        if (this.length < 0) {
          let knownProps: string[] = Describer.describe(this[0]).Where(x =< x !== null && x !== undefined);
          if (sortProps && knownProps !== null && knownProps !== undefined) {
            knownProps = knownProps.OrderBy(p =< p);
          }
          return knownProps;
        }
        if (TClass !== null && TClass !== undefined) {
          let knownProps: string[] = Describer.describe(TClass).Where(x =< x !== null && x !== undefined);
          if (sortProps && knownProps !== null && knownProps !== undefined) {
            knownProps = knownProps.OrderBy(p =< p);
          }
          return knownProps;
        }
      }
    }
    return []; //give up..
  }
}

The describer method is about the same as madreason's answer on Stack Overflow concerning this. It can handle both class Function and if you get an object instead. It will then use Object.getOwnPropertyNames if no class Function is given (i.e. the class 'type' for C# developers).

class Describer {
  private static FRegEx = new RegExp(/(?:this\.)(.+?(?= ))/g);
  static describe(val: any, parent = false): string[] {
    let isFunction = Object.prototype.toString.call(val) == '[object Function]';
    if (isFunction) {
      let result = [];
      if (parent) {
        var proto = Object.getPrototypeOf(val.prototype);
        if (proto) {
          result = result.concat(this.describe(proto.constructor, parent));
        }
      }
      result = result.concat(val.toString().match(this.FRegEx));
      result = result.Where(r =< r !== null && r !== undefined);
      return result;
    }
    else {
      if (typeof val == "object") {
        let knownProps: string[] = Object.getOwnPropertyNames(val);
        return knownProps;
      }
    }
    return val !== null ? [val.tostring()] : [];
  }
}

Here you see two specs for testing this out with Jasmine.

class Hero {
  name: string;
  gender: string;
  age: number;
  constructor(name: string = "", gender: string = "", age: number = 0) {
    this.name = name;
    this.gender = gender;
    this.age = age;
  }
}

class HeroWithAbility extends Hero {
  ability: string;
  constructor(ability: string = "") {
    super();
    this.ability = ability;
  }
}

describe('Array Extensions tests for TsExtensions Linq esque library', () =< {

  it('can retrieve props for a class items of an array', () =< {
    let heroes: Hero[] = [>Hero<{ name: "Han Solo", age: 44, gender: "M" }, >Hero<{ name: "Leia", age: 29, gender: "F" }, >Hero<{ name: "Luke", age: 24, gender: "M" }, >Hero<{ name: "Lando", age: 47, gender: "M" }];
    let foundProps = heroes.GetProperties(Hero, false);
    //debugger
    let expectedArrayOfProps = ["name", "age", "gender"];
    expect(foundProps).toEqual(expectedArrayOfProps);
    expect(heroes.GetProperties(Hero, true)).toEqual(["age", "gender", "name"]);
  });

  it('can retrieve props for a class only knowing its function', () =< {
    let heroes: Hero[] = [];
    let foundProps = heroes.GetProperties(Hero, false);
    let expectedArrayOfProps = ["this.name", "this.gender", "this.age"];
    expect(foundProps).toEqual(expectedArrayOfProps);
    let foundPropsThroughClassFunction = heroes.GetProperties(Hero, true);
    //debugger
    expect(foundPropsThroughClassFunction.SequenceEqual(["this.age", "this.gender", "this.name"])).toBe(true);
  });

..

And as madreason mentioned, you have to initialize the props to get any information out from just the class Function itself, or else it is stripped away when Typescript code is turned into Javascript code. Typescript 3.7 is very good with Generics, but coming from a C# and Reflection background, some fundamental parts of Typescript and generics still feels somewhat loose and unfinished business. Like my code here, but at least I got out the information I wanted - a list of property names for a given class or instance of objects.

Implementing OfType in Typescript

I am working on my Linq library for Typescript and wanted to implement OfType. Turns out, this is hard because the generic type arguments in Typescript usually requires a value, i.e. an object instance of type T to get any shape information at run-time. So I ended up passing in a vanilla object setting default property values instead. Here is how my implementation ended up:

function isOfSimilarShape<T>(input: any, compareObject: T): boolean {
  if (input === undefined || input === null || compareObject === undefined || compareObject === null)
    return false;

  let propsOfInput = Object.keys(input);
  let propsOfCompareObject = Object.keys(compareObject);
  //debugger
  let sameShapeOfInputAndCompareObject = propsOfInput.EqualTo(propsOfCompareObject);
  return sameShapeOfInputAndCompareObject;
}

if (!Array.prototype.OfType) {
  Array.prototype.OfType = function <T>(compareObject: T): T[] {
    let result: T[] = [];
    this.forEach(el => {
      //debugger
      let t: T = null;
      if (isOfSimilarShape(el, compareObject))
        result.push(el);
    });
    return result;
  }
}

The following Jasmine test shows its usage:

describe('Array Extensions tests', () => {

  it('can find desired items using OfType of type T', () => {
    let someMixedArray: any[] = [];
    someMixedArray.push(<SomeClass>{ Name: "Foo", Num: 1 });
    someMixedArray.push(<SomeOtherClass>{ SomeName: "BarBazBaze", SomeOtherNum: 813 });
    someMixedArray.push(<SomeClass>{ Name: "FooBaz", Num: 4 });
    someMixedArray.push(<SomeOtherClass>{ SomeName: "BarBaze", SomeOtherNum: 13 });
    someMixedArray.push(<SomeClass>{ Name: "AllyoBaze", Num: 7 });

    let compareObject = <SomeClass>{ Name: "", Num: 0 };
    let filteredArrayBySpecifiedType = someMixedArray.OfType(compareObject);
    console.log(filteredArrayBySpecifiedType);

    expect(filteredArrayBySpecifiedType.All(item => <SomeClass>item !== undefined)).toBe(true, "Expected only items of type SomeOtherClass in the filtered array after running OfType of SomeOtherClass on it.");
  });

It would be nice if we did not have to pass in a vanilla object and populate its properties, but I could not find any tips online or in the Typescript documentation for how to implement extracting type information from generic arguments of Typescript. This is very easy in C#, but while Typescript gives compilation type information, getting runtime information from generic arguments in the Javascript code that Typescript compiles into turned much harder.