Edit in JSFiddle

var store = mobx.observable({
    counter: 0
})

function render(state) {
    document.getElementById('counter').textContent = state.counter;
}

document.getElementById('button').addEventListener('click', function() {
    store.counter = store.counter + 1
})

mobx.observe(store, function() {
    render(store)
})

<div id="counter">-</div>
<button id="button">Increment</button>