JavaScript is a prototype-based, multi-paradigm, single-threaded, dynamic language, supporting object-oriented, imperative, and declarative (e.g. functional programming) styles.
Created at Netscape for making dynamic webpages, now used for server backends too because of JS Engines like V8 running outside a browser like in Node.js.
The language is standardized by Ecma International as ECMAScript Language Specification. Javascript is the most popular implementation of ECMAScript.
It is a JIT compiled interpreted language.
Many new languages are transpiled (converted) to Javascript before they run in the browser. Ex - TypeScript (Microsoft), Flow (Facebook), Dart (Google).
Deno was created by the creator of Node.js Ryan Dahl in 2018, based on V8 engine.
<!-- use script tags anywhere in HTML -->
<script>
alert("A");
</script>
<script>
alert("B");
</script>
<!-- or use external JS file -->
<script src="my.js">
// any JS code written here will be ignored when using external script (src="")
</script>
# run with Node
$> node hello.js
# run with Deno
$> deno run hello.js