JSFiddle - React, Tailwind, and code Playground

HTML

Categories:
<select data-bind='options: exerciseCategories , optionsText: "category", optionsCaption: "Izberite...", value: selectedCategory'></select>
<p>selectedCategory() debug: <pre data-bind="text: selectedCategory() ? ko.toJSON(selectedCategory().exercises, null, 2) : ''"></pre>
</p>Exercises:
<select data-bind='options: selectedCategory() ? selectedCategory().exercises : [], optionsText: "title", value: selectedExercise'></select>
<p>selectedExercise() debug: <pre data-bind="text: selectedExercise() ? ko.toJSON(selectedExercise(), null, 2) : 'x'"></pre>
</p>
<input type="text" data-bind="attr : { value : selectedExercise() ? selectedExercise().intervals : 0 }"/>

JavaScript

var exerciseCategories = [{
    exercises: [{
        title: 'Aerobic exercise #1',
        intervals: 2
    }],
    category: 'Aerobics'
}];

var ExerciseViewModel = function () {
    var self = this;

    self.exerciseCategories = ko.observable(exerciseCategories);
    self.selectedCategory = ko.observable();
    self.selectedExercise = ko.observable();
    self.intervals = ko.observable(1);
};
ko.applyBindings(new ExerciseViewModel());