Creating model classes in TypeScript

Yes, you can do it.

Step 1: Create your model using “Classes”. While TypeScript has interfaces that can provide this functionality, the Angular team recommends just using a bare ES6 class with strongly typed instance variables. ES6 classes allow you to (optionally) build out functionality around your models and also doesn't require you to be locked into a TypeScript specific feature. For these reasons, it's advisable to use classes for creating models.

export class DonutChartModel {

//Fields 
dimension: Number
innerRadius: Number
backgroundClass: Number
backgroundOpacity: Number
myPropertyToSet: String 

constructor (dimension: Number, innerRadius: Number){
   this.dimension = dimension
   this.innerRadius = innerRadius
}}

Step 2: Import it into your component. This will give you the added benefit of reuse the data model in multiple places.

import { DonutChartModel } from '../../models/donut-chart-model;

Step 3: Set one of the properties values:

export class MenuSelectionPage {

 myDonuts: DonutChartModel[] = [];

 constructor(public navCtrl: NavController, public navParams: NavParams) {
  this.FillLocalData()
  this.myDonuts[this.myDonuts.length - 1].myPropertyToSet = "I am your father" 
 } 

//Aux Methods
FillLocalData() {
let dimensions = [8.32, 5, 17];
let defaultInnerRadius = 2;
for (let i = 0; i < dimensions.length; i++) {
  let donut = new DonutChartModel (dimensions[i], defaultInnerRadius * i)
  this.myDonuts.push(donut)
}}}

Step 4 (Optional): Use it in html.

 ...
 <ion-list>
    <button ion-item *ngFor="let donut of myDonuts">
    {{donut.myPropertyToSet}}
     </button>
 </ion-list>
 ...

Note: This code has been tested in ionic 3

What it appears you are attempting to accomplish is to enforce a structure on a model. While it makes sense to use a class in C# to accomplish this, in TypeScript the best approach is to create either a type or an interface.

Here are examples of both (reduced properties for brevity)

Type

type DonutChartModel = {
    dimension: number;
    innerRadius: number;
};
var donut: DonutChartModel = {
    dimension: 1,
    innerRadius: 2
};

Interface

interface IDonutChartModel {
    dimension: number;
    innerRadius: number;
}
var donut: IDonutChartModel = {
    dimension: 1,
    innerRadius: 2
};

When to Use:

Interfaces can be extended from/by classes and are best for when you are defining properties.

Types can be combined and should be used more for non-composite properties. A good example would be to use types for something like this:

type Direction = 'up' | 'down' | 'left' | 'right';

An excellent resource on types can be found here, or as answers to TypeScript: Interfaces vs Types.

Giving the fields default values should do what you're looking for.

export class DonutChartModel {
    dimension: number = 0;
    innerRadius: number = 0;
    backgroundClass: string = "";
    backgroundOpacity: number = 0;
}