Supported Javascript Functions in Applescript

Where can I find the list of supported javascript function in Applescript?
I.E. string.startsWith() is not supported in Yosemite, but it is supported in El Capitan.

When you say javascript ‘in AppleScript’, do you mean JavaScript for Automation ?

JavaScript for Automation (JXA) is the Safari JavaScript compiler embedded in a special JSContext (outside the browser) with one special Library object called Automation.

For the String, Array etc methods supported by the JavaScript compiler, you need to look up the JavaScript of the Safari version running on the OS X version which interests you. For example, you can look up the SF8 SF9 and SF10 columns here:

Awesome! Thanks for the link!