This JavaScript editing mode supports:

 - strict recognition of the Ecma-262 language standard
 - support for most Rhino and SpiderMonkey extensions from 1.5 and up
 - parsing support for ECMAScript for XML (E4X, ECMA-357)
 - accurate syntax highlighting using a recursive-descent parser
 - on-the-fly reporting of syntax errors and strict-mode warnings
 - undeclared-variable warnings using a configurable externs framework
 - "bouncing" line indentation to choose among alternate indentation points
 - smart line-wrapping within comments and strings
 - code folding:
   - show some or all function bodies as {...}
   - show some or all block comments as /*...*/
 - context-sensitive menu bar and popup menus
 - code browsing using the `imenu' package
 - many customization options

Installation:

To install it as your major mode for JavaScript editing:

  (add-to-list 'auto-mode-alist '("\\.js\\'" . js2-mode))

Alternatively, to install it as a minor mode just for JavaScript linting,
you must add it to the appropriate major-mode hook.  Normally this would be:

  (add-hook 'js-mode-hook 'js2-minor-mode)

You may also want to hook it in for shell scripts running via node.js:

  (add-to-list 'interpreter-mode-alist '("node" . js2-mode))

Use Emacs 27 and want to write JSX?  Then use `js2-minor-mode' as described
above.  Use Emacs 26 or earlier?  Then use `js2-jsx-mode':

  (add-to-list 'auto-mode-alist '("\\.jsx?\\'" . js2-jsx-mode))
  (add-to-list 'interpreter-mode-alist '("node" . js2-jsx-mode))

Note that linting of JSX code may fail in both modes.

To customize how it works:
  M-x customize-group RET js2-mode RET

Notes:

This mode includes a port of Mozilla Rhino's scanner, parser and
symbol table.  Ideally it should stay in sync with Rhino, keeping
`js2-mode' current as the EcmaScript language standard evolves.

Unlike cc-engine based language modes, js2-mode's line-indentation is not
customizable.  It is a surprising amount of work to support customizable
indentation.  The current compromise is that the tab key lets you cycle among
various likely indentation points, similar to the behavior of python-mode.

This mode does not yet work with "multi-mode" modes such as `mmm-mode'
and `mumamo', although it could be made to do so with some effort.
This means that `js2-mode' is currently only useful for editing JavaScript
files, and not for editing JavaScript within <script> tags or templates.

The project page on GitHub is used for development and issue tracking.
The original homepage at Google Code has outdated information and is mostly
unmaintained.