how do web crawlers handle javascript

JavaScript is handled by both Bing and Google crawlers. Yahoo uses the Bing crawler data, so it should be handled as well. I didn't look into other search engines, so if you care about them, you should look them up.

Bing published guidance in March 2014 as to how to create JavaScript-based websites that work with their crawler (mostly related to pushState) that are good practices in general:

• Avoid creating broken links with pushState
• Avoid creating two different links that link to the same content with pushState
• Avoid cloaking. (Here's an article Bing published about their cloaking detection in 2007)
• Support browsers (and crawlers) that can't handle pushState.

Google later published guidance in May 2014 as to how to create JavaScript-based websites that work with their crawler, and their recommendations are also recommended:

• Don't block the JavaScript (and CSS) in the robots.txt file.
• Make sure you can handle the load of the crawlers.
• It's a good idea to support browsers and crawlers that can't handle (or users and organizations that won't allow) JavaScript
• Tricky JavaScript that relies on arcane or specific features of the language might not work with the crawlers.
• If your JavaScript removes content from the page, it might not get indexed. around.