leporello-js/docs/examples/preact/index.js

const fib = n => {
  if(n == 0) {
    return 0
  }
  if(n == 1) {
    return 1
  }
  return fib(n - 1) + fib(n - 2)
}
    
/* external */
import {h, render} from 'https://unpkg.com/preact?module';

/* external */
import {Stateful} from './stateful.js'

const Fibonacci = Stateful({
  getInitialState: () => ({index: 0}),

  handlers: {
    prev: ({index}, event) => ({index: index - 1}),
    next: ({index}, event) => ({index: index + 1}),
  },
  
  render: (props, state, handlers) => 
    h('div', null,
      h('h1', null, 
        'nth Fibonacci number is ', 
        fib(state.index),
        ' for n = ', 
        state.index
      ),
      h('button', {onClick: handlers.prev}, 'Previous'), ' ',
      h('button', {onClick: handlers.next}, 'Next'), ' ',
    )
})

render(h(Fibonacci), globalThis.document.body)