not defined - garevna/js-course GitHub Wiki

☕ 1️⃣

<body>
    <hello-element></hello-element>
    <bye-element></bye-element>
</body>

document.querySelectorAll ( ":not(:defined)" )

▶ NodeList(2) [hello-element, bye-element]

document.querySelectorAll ( ":defined" )

▶ NodeList(3) [html, head, body]