dragsugur please always start a new support discussion. This is completely unrelated to the other user's problem.
You need NPM and Webpack to work to make changes to javascript. You should fork the extension you are modifying and clone the fork in a composer local "workbench" folder to preserve your changes when you update Flarum or other extensions.
The process to compile the javascript of extensions is described here https://docs.flarum.org/extend/frontend
If you modify a dist file (rather than the source) under vendor and clear the Flarum cache, it should work, but the changes will be lost the next time you update Flarum or an extension.
If you're trying to change something from Flarum core, it's recommended to create a custom extension that modifies core behavior instead of forking core directly, because forking core or a core extension will require more work in the long run to keep it up to date with Flarum changes.
