Can user directly modify client sdie code and bypass checks?

sorry for the newbie question .

i am developing a react/next.js website , so i start to wonder what if user just change the client side code ?

for example, i fetch data from server , if it returns ok then user can proceeed to next page , if not then user gets an error.

In this scenario , what if user just change the if statement to true which would make my fetch authentication useless .

Thank you .