Here is a list of some of the functions that you can use specifically in client side scripts.
Some functions are exclusive to the scripting runtime you're using, and are not documented in the FiveM Native Reference List. Refer to their docs for more detail.
These are native functions provided by both the Citizen framework (under the CFX heading), as well as by the game. Clicking each link will lead to the FiveM native reference, where you can see syntax per language, a description, and examples for using the native.
These natives are usable in all runtimes.