tils-first-weather-widget/src/model/forecast.ts

An example development EXB widget (Grant Caroll, spatial-innovation.co.nz)

import Accessor from "@arcgis/core/core/Accessor.js";
import { property } from "@arcgis/core/core/accessorSupport/decorators.js";
import { IForecastDay } from "./forecast-day";

export interface IForecast {
    forecastday: Array<IForecastDay>;
}

export default class Forecast extends Accessor implements IForecast{

    @property()
    forecastday: Array<IForecastDay>;

    constructor(params: IForecast) {
        super();
        this.forecastday = params.forecastday;
    }

}