The poor man's SolidJS implemented in 40 lines

solid.userscript.js

const $  = document.querySelector.bind(document),
      $$ = document.querySelectorAll.bind(document),
      h  = (name, props) => Object.assign(document.createElement(name), props);

const context = []
const getCurrentObserver = () => context[context.length - 1]

function createEffect(fn, name) {
  const execute = () => {
    context.push(execute)
    try {
      fn()
    } finally {
      context.pop()
    }
  }

  execute()
}

function createSignal(value) {
  const subscribers = new Set()

  const read = () => {
    const current = getCurrentObserver()
    if (current) subscribers.add(current)
    return value
  }

  const write = (nextValue) => {
    if (typeof nextValue === 'function') {
      value = nextValue(value)
    } else {
      value = nextValue
    }
    for (const sub of subscribers) sub()
  }

  return [read, write]
}