mithril-updateEvent-2.js

function updateEvent(vnode, key, value) {
  if (vnode.events != null) {
    if (vnode.events[key] === value) return
    if (value != null && (typeof value === "function" || typeof value === "object")) {
      if (vnode.events[key] == null) vnode.dom.addEventListener(key.slice(2), vnode.events, false)
      vnode.events[key] = value
    }
    // ...
  }
  // ...
}