You are building an application running inside the web browser (a classic SPA). You have to read a list of user's follow

[answerhappygod]

You are building an application running inside the web browser
(a classic SPA). You have to read a list of user's followers'
usernames from the server. This list can contain thousands of
usernames. You need to implement a component that finds a username
from the list based on user input. For example given a list of
usernames 'fred', 'frank', 'jerry' . When a user types fr , you
should filter the list so that usernames starting with fr are
included ( fred and frank in this case).
How would you implement this (data structures and
algorithms)?