• Sample tests do not compile because you forgot the following imports

  import { parseRedirects } from './solution';
  import * as assert from 'assert'
  

• Only 1 test and no random tests

• No mention whatsoever about the output format (regarding the source array)

• Multiple sources must be grouped together into one object.

  There must never be more than one object with the same destination.

  Grouping is not based on the source but the destination

You could use .padStart(2, '0') method instead of locale