I am currently developing an extension, when i comment code of the extension, i relise it doesn't disappear it's still on the web, can somebody explains for me what is happening

If you want help with this, you are going to need to provide a lot more detail. What do you mean by “extension”? When you say you comment the code but it is still “on the web”, what do you mean by that? Are developing locally or in Umbraco Cloud or in Azure?

it means i am creating a custom extension (vanilla JS) and when it delete code of it, it's still available on web and i am developing on local

What version of Umbraco are you using? And by "custom extension" do you mean a backoffice component like a property editor or dashboard, or something for the front-end? If this is for a backoffice component, and if you are using a version between v9 and v13, you likely just need to hard refresh your browser (ctrl-f5). Also, ensure that your js changes are getting deployed to the correct location when you build your project. If you are using a version < v9, you might need to manually change the "version" number in the /config/ClientDependency.config. This shouldn't strictly be necessary, but I have found that it often was when backoffice changes weren't getting picked up correctly by Umbraco, due to the backoffice caching the JS, html, and css.

Yeah that's it and i am using newest version, when i change my code it doesn't changes immediately, i wonder if i need to clear cache to apply new changes