Given typescript():
  let abc = y.
  let foo = ab
  let foo = (ab)

Execute(Completion should be done after dots in TypeScript):
  AssertEqual '.', ale#completion#GetPrefix(&filetype, 1, 13)

Execute(Completion should be done after words in TypeScript):
  AssertEqual 'ab', ale#completion#GetPrefix(&filetype, 2, 13)

Execute(Completion should be done after words in parens in TypeScript):
  AssertEqual 'ab', ale#completion#GetPrefix(&filetype, 3, 14)

Execute(Completion should not be done after parens in TypeScript):
  AssertEqual '', ale#completion#GetPrefix(&filetype, 3, 15)

Execute(Completion prefixes should work for other filetypes):
  AssertEqual 'ab', ale#completion#GetPrefix('xxxyyyzzz', 3, 14)