You can use the following endpoint/link with HttpService to retrieve the latest api dump version.
http://setup.roblox.com/versionQTStudio
You can then use this version to request the appropriate JSON api dump from this other link, but replace VERSION with the retrieved string.
http://setup.roblox.com/VERSION-API-Dump.json
The dump should contain everything from property names and types to (I believe) methods.
(They’re Roblox subdomains so you might need a proxy?)