[Style Guide] When should you add on- or handle-?

Question

JavaScript

I have no problems with the naming of functions, it is quite simple here.

Functions starting with…
"get…" return a value,
"calc…" compute something,
"create…" create something,
"check…" check something and return a boolean value, etc. d.

But in what cases should you add the on- or handle- prefix?
Are prefixes needed? It was not possible to find information about this

Now I call it that.
Let's assume that the method generates a contacts object, then I'll call it - formatObjectContacts.
If it does something else before formatting the object, then onFormatObjectContacts.

Am I going the right way? Tell us what rules you use or poke your nose into some article about this

(Learning Vue.js)