Helpful Links

JavaScript:

Useful:

• http://jquery.com/
• http://jsfiddle.net/
• http://www.jslint.com/
• http://requirejs.org/
• http://cdnjs.com/

Performance:

• http://jsperf.com/
• http://developer.yahoo.com/performance/rules.html
• http://oreilly.com/server-administration/excerpts/even-faster-websites/writing-efficient-javascript.html
• http://jonraasch.com/blog/10-javascript-performance-boosting-tips-from-nicholas-zakas
• http://www.artzstudio.com/2009/04/jquery-performance-rules/

Specification:

• https://developer.mozilla.org/en-US/docs/JavaScript/Language_Resources
• http://www.ecma-international.org/publications/standards/Ecma-262.htm

Tutorials / Guides:

• http://bonsaiden.github.io/JavaScript-Garden/
• http://superherojs.com/
• https://developer.mozilla.org/en-US/docs/JavaScript/Guide
• http://yuiblog.com/crockford/

Editors:

• http://notepad-plus-plus.org/
• http://www.vim.org/
• http://www.eclipse.org/
• http://www.sublimetext.com/
• http://www.ultraedit.com/

Other:

• http://overapi.com/javascript/
• http://paulirish.com/2010/10-things-i-learned-from-the-jquery-source/
• http://paulirish.com/2011/11-more-things-i-learned-from-the-jquery-source/
• http://www.cambiaresearch.com/articles/15/javascript-char-codes-key-codes
• http://addyosmani.com/resources/essentialjsdesignpatterns/book/
• http://net.tutsplus.com/tutorials/javascript-ajax/principles-of-maintainable-javascript/
• http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml

Also, be aware that it is recommended that you do not use W3Schools:

W3Schools frequently publishes inaccurate or misleading content.

Use resources like mdn instead.