PureComponent.js

class PureComponent {
    initializeEvent(element, name, expession) {
        let func = this[name];
        let handler = (e) => {
            let newState = func(this.state, parseExpressionToArgs(expression, state, e));
            this.render(newState);
        }
        element.addEventListener(name, handler, false)
    }
}

function parseExpressionToArgs(expression, state, e) {
    ...
}

PureSpinner.js

class PureSpinner extends PureComponent {
    render(state) {
        let template = '<input /><span on-click="increment">UP</span><span on-click="decrement">DOWN</span>';
        return this.renderWithTemplate(template, state);
    }
    
    increment(state) {
        // Returns new state
        return {...state, value: state.value + state.step};
    }
    
    decrement(state) {
        // Returns state patch
        return {value: state.value - state.step};
    }
}

spec.js

let spinner = new Spinner();
let newState = spinner.increment({value: 1, step: 2});
expect(newState).toBe(3);
let {newValue} = spinner.increment({...newState, step: 3});
expect(newValue).toBe(6);