Get class (interface) properties in Angular 5 / TypeScript without assigning a default value

Question

Let's pretend that I have an interface A, that I am declaring as a class by following the Angular style guide. This class has many properties, and I want to fetch them names without having to assign any value to them. How can I achieve this ?

Class A:

export class A {
  property1: string;
  property2: string;
  ...
  property30: string;
}

I tried with instantiating a new object from this class and calling Object.keys and Object.getOwnPropertyNames but this two methods return an empty array because they are ignoring undefined value properties. Is there any way to bypass this behaviour ? Or am I breaking the JavaScript/TypeScript pattern ? :D

Possible duplicate of [Get properties of a class using Typescript](https://stackoverflow.com/questions/40636292/get-properties-of-a-class-using-typescript) — ochs.tobi, Mar 08 '18 at 13:55
Nop it's not a duplicate because in the link provided they are giving some default values to the properties. — Mehdi Benmoha, Mar 08 '18 at 14:16

score 3 · Accepted Answer · answered Mar 08 '18 at 13:55

3

The way properties declarations work is that they are just a hint to the compiler that that property may exist at run-time. In JavaScript you don't need to declare fields, so until the field is assigned it will not exist on the object. If you initialize the field even just with null or undefined the field will appear on the object. This is the simplest way to achieve what you want.

The other way would be to use a decorator on every field. This would be more explicit but not shorter and not necessarily less error prone

answered Mar 08 '18 at 13:55

Titian Cernicova-Dragomir

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The thing is that I have a lot of classes for which I want to know the properties, and I can't do that without assigning a default value. Right ? – Mehdi Benmoha Mar 08 '18 at 14:10
@e.m.b In either solution, assigning a default or decorators, you will have to do something to all properties – Titian Cernicova-Dragomir Mar 08 '18 at 15:21

windmaomao · Answer 2 · 2018-03-08T14:22:44.663

0

Make sure your compiler supports class first. And then you have to do either interface or class. Ex. if it's a class, you need to do

class A {
    title = ''
}
var a = new A();
console.log(a);

declare title as a string, title: String = ''. Your writing is more for the interface. Here's the output,

Object {
    title: ""
}

Yeah, I understand your question better now, without assignment, it doesn't set the properties.

interface B {
    title: String;
}
class A implements B {
    title = '';
}
var a = new A();
console.log(a);

If that's the case, a simple solution is to give some default value of that property. Or going through a list to manually initialize all of them based by their type.

class A {
    keys = {
        title: 'String'
    }
}

edited Mar 08 '18 at 14:22

answered Mar 08 '18 at 14:00

windmaomao

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There is no way without setting a default value ? – Mehdi Benmoha Mar 08 '18 at 14:07
Thanks e.m.b, interesting, this is just so interesting. – windmaomao Mar 08 '18 at 14:11
The thing is that I have a lot of classes and I am too lazy like every developer lol to assign a default value for every class properties. – Mehdi Benmoha Mar 08 '18 at 14:12
try `a.constructor`, ex. `console.log(a.constructor)` from the above example – windmaomao Mar 08 '18 at 14:14
a.constructor return a function called A() that takes 0 parameters and I only have an attribute name equal to A. – Mehdi Benmoha Mar 08 '18 at 14:17
you're right, sorry, that doesn't work either. Ok, I gave up :) Here's fallback, `class A { title = new String(); }`. I think the missing link is a manual initialization based on the `type`. I don't know Javascript actually doesn't do that!! – windmaomao Mar 08 '18 at 14:19

Get class (interface) properties in Angular 5 / TypeScript without assigning a default value

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