Reading property names from a class in TypeScript?

Question

Having a class (e.g. WikipediaSearchResult) with a few properties in TypeScript, is it possible to access them? Maybe this question is a bit naive, but I'm wondering if I could write following code without duplicated property names:

function mergeTo(from:any, to:any={}, properties:Array<string>=[]) {
  if (!!from) {
    for (var property of properties) {
      // no deep copy here ;-)
      to[property] = from[property];
    }
  }
}

class WikipediaSearchResult /* implements IWikipediaSearchResult */ {

  lang:string;
  summary:string;
  title:string;
  wikipediaUrl:string;

  constructor(obj?:any) {

    mergeTo(obj, this, [
      // --> How to avoid this list? <--
      'lang', 'summary', 'title', 'wikipediaUrl'
    ]);

  }

}

var result = new WikipediaSearchResult({
  title: 'Zürich', 
  wikipediaUrl: 'https://en.wikipedia.org/wiki/Z%C3%BCrich'
});

console.log(result);

Of course there are some 3rd party libraries such as Underscore.js, but it differs from e.g. _.clone(...) since I do only want to clone specific properties and ignoring all other properties which may be provided by obj respectively from.

Another way might be using e.g. _.create(WikipediaSearchResult.prototype, { title: 'Zürich' }), haven't tried it yet but it will use the JavaScript prototype property. It would be great to use TypeScript internal mechanism.

An idea which I came up with is to create a "dummy" instance and reading its key. I'm wondering if someone comes up with better variant?

why don't you iterate over from? for (var property in from) { to[property] = from[property]; } — ichramm
– ichramm, Commented Jan 27, 2016 at 13:52
from (any) might have more properties which should not be set in to — Thomas
– Thomas, Commented Jan 27, 2016 at 13:53

Thomas · Accepted Answer · 2016-01-27 15:13:56Z

1

Initializing the properties of class WikipediaSearchResult allows using this as a "template":

function mergeTo(from:any, to:any={}, properties:Array<string>=[]) {
  // same as in question
}

function mergeTo2(from:Object, to:Object):Object {
  // _.keys from underscore
  return mergeTo(from, to, _.keys(to));
}

class WikipediaSearchResult {

  lang:string = undefined;
  summary:string = undefined;
  title:string = undefined;
  wikipediaUrl:string = undefined;

  constructor(obj?:any) {
     util.mergeTo2(obj, this);
  }

}

var result = new WikipediaSearchResult({
  title: 'Zürich', 
  wikipediaUrl: 'https://en.wikipedia.org/wiki/Z%C3%BCrich'
});

console.log(result);

Still looking for other variants...

answered Jan 27, 2016 at 15:13

Thomas

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Amid · Accepted Answer · 2016-01-27 15:20:46Z

1

You can apply to each of the copiable properties a custom decorator:

  @Copiable
  lang:string;

Then inside @copiable decorator function you will store target property name in some static collection.

function Copiable(target: any, key: string) 
{
    CopiableProperties.push(key); 
}

Then later in mergeTo method you will be able to iterate over all copiable property names stored in CopiableProperties.

It might be a bit overkill but it will allow you to avoid duplicates. Hope this helps.

answered Jan 27, 2016 at 15:20

Amid

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3 Comments

Thomas Over a year ago

Interesting approach but as you stated it's a bit overkill. And another question might arise: Where to store this collection?

Amid Over a year ago

For my purposes I have a special public helper static class dedicated to contain such 'meta' information.

Thomas Over a year ago

I'll like the idea of using decorators(!) but it does not help to make the code more readable or maintanable. I rather go with my answer because of these reasons. Bring me an argument why your solution is better than mine and I am going to change the accepted answer to yours. ;)

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