I have the API route /api/users/:user_id, which takes a number as a parameter.
I want to have now the same route, but taking the username instead of the id, like this: /api/users/:username.
In my code, I have the route set up as /api/users/:user and I check the user URI parameter and do different actions if it is a number or a string.
Is it good practice / efficient to have the same route, but with a different parameter type? Both the username and user id's are unique.
It works, I just want to know if this is a practical solution, or if I should do separate routes.
It isn't really "good practice" to share a route parameter on a route, but as long as the IDs and usernames are both unique, you should be fine. Both act as unique identifiers for a user so both can be used to find the user in that route.
You can accept both the ID and username as the same parameter by first making the route param more permissive. Next, you can use the following (pseudo) query to look up whether that param matches the ID or the username:
SELECT id, name FROM users WHERE id={param} OR username={param}
Remember to pass that param in as a real query parameter; do NOT simply concatenate strings. Doing so will open you up to an SQL injection attack.