steps to add javascript code (JSOM) in sharepoint 2016 (On-Prem) page

Just like with CSS, JavaScript cannot be added by editing the HTML source of the page directly. You have to use the Content Editor web part:

- Add a Content Editor web part to the page.

- (Click on the link to add new content, if you are using a web part page.)

- Click on the Edit Source button in the ribbon.

- Paste your JavaScript code in the HTML source.

- Apply and save the page

 

Check Step by step

https://support.shortpoint.com/support/solutions/articles/1000265208-how-to-add-javascript-code-to-a-page-using-the-script-editor-web-part

 

For JSOM Code:

https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/complete-basic-operations-using-javascript-library-code-in-sharepoint