Keep consistent in handling indexOf results

What does this MR do?

Keeps consistent in handling indexOf results in Javascript.

~"technical debt", javascript,

Are there points in the code the reviewer needs to double check?

Only javascript files in app are fixed while vendored javascript files are unchanged.

Why was this MR needed?

To save time to discuss bikeshed.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added
• [n/a] Documentation created/updated
• [na] API support added
• Tests
  • [n/a] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28723 (closed)