Update usage of TypeScript

[victorlin]

Description of proposed changes

Small improvements suggested by others.

Related issue(s)

Follow-up to:

• 874

• https://github.com/nextstrain/nextstrain.org/pull/901#discussion_r1632403960
• Slack thread

Checklist

• [x] Checks pass
• [x] ~Check if changes affect the resource index JSON revision~