snabbdom/modules/attributes.js

function isBooleanAttribute(attrName) {
  return (/^(?:allowfullscreen|async|autofocus|autoplay|checked|compact|controls|declare|default|defaultchecked|defaultmuted|defaultselected|defer|disabled|draggable|enabled|formnovalidate|hidden|indeterminate|inert|ismap|itemscope|loop|multiple|muted|nohref|noresize|noshade|novalidate|nowrap|open|pauseonexit|readonly|required|reversed|scoped|seamless|selected|sortable|spellcheck|translate|truespeed|typemustmatch|visible)$/).test(attrName);
}

function updateAttrs(oldVnode, vnode) {
  var key, cur, old, elm = vnode.elm,
      oldAttrs = oldVnode.data.attrs || {}, attrs = vnode.data.attrs || {};
  
  // update modified attributes, add new attributes
  for (key in attrs) {
    cur = attrs[key];
    old = oldAttrs[key];
    if (old !== cur) {
      // TODO: add support to namespaced attributes (setAttributeNS)
      if(!cur && isBooleanAttribute(key))
        elm.removeAttribute(key);
      else
        elm.setAttribute(key, cur);
    }
  }
  //remove removed attributes
  // use `in` operator since the previous `for` iteration uses it (.i.e. add even attributes with undefined value)
  // the other option is to remove all attributes with value == undefined
  for (key in oldAttrs) {
    if (!(key in attrs)) {
      elm.removeAttribute(key);
    }
  }
}

module.exports = {create: updateAttrs, update: updateAttrs};