Here is a list of some of the functions that you can use specifically in client side scripts.
Native functions are provided by both the game and the Citizen framework (under the CFX heading). Refer to the FiveM Native Reference, where you can see syntax per language, a description, and examples for each native.
These natives are usable in all runtimes.
Runtime specific functions
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.