How to implement watcher for data?

Question

JavaScript

On the report, the guy says that vue somehow puts a native watcher function for each data.
Link to youtube with time reference
I just can't find the native watcher function, maybe someone knows?
I want to call the rerender function when the object changes. I'm not writing in vue , but in pure. I know that there are watchers in the view, but I have a project without vue.