undefined - JavaScript | MDN
The undefined global property represents the primitive value undefined. It is one of JavaScript's primitive types.| MDN Web Docs
We are an open community of developers, technical writers, and learners building resources for a better Web, regardless of brand, browser, or platform. Anyone can contribute, and each person who does contribute has an impact on millions of readers. Learn how to contribute and drive innovation on the Open Web.| developer.mozilla.org
The undefined global property represents the primitive value undefined. It is one of JavaScript's primitive types.| MDN Web Docs
HTML| html.spec.whatwg.org
