Add `babylon` as default Prettier parser (#2220)

* Mimic Prettier's default parser by setting it to `babylon`
* Add tests to check default Prettier `parser`
* Set Prettier default parser based on version
* Update the comment to explain the reason for an explicit default

autoload/ale/fixers/prettier.vim

@@ -47,6 +47,15 @@ function! ale#fixers#prettier#ApplyFixForVersion(buffer, version_output) abort
     " Append the --parser flag depending on the current filetype (unless it's
     " already set in g:javascript_prettier_options).
     if empty(expand('#' . a:buffer . ':e')) && match(l:options, '--parser') == -1
+        " Mimic Prettier's defaults. In cases without a file extension or
+        " filetype (scratch buffer), Prettier needs `parser` set to know how
+        " to process the buffer.
+        if ale#semver#GTE(l:version, [1, 16, 0])
+          let l:parser = 'babel'
+        else
+          let l:parser = 'babylon'
+        endif
+
         let l:prettier_parsers = {
         \    'typescript': 'typescript',
         \    'css': 'css',
@@ -60,7 +69,6 @@ function! ale#fixers#prettier#ApplyFixForVersion(buffer, version_output) abort
         \    'yaml': 'yaml',
         \    'html': 'html',
         \}
-        let l:parser = ''
 
         for l:filetype in split(getbufvar(a:buffer, '&filetype'), '\.')
             if has_key(l:prettier_parsers, l:filetype)

test/fixers/test_prettier_fixer_callback.vader

@@ -96,6 +96,34 @@ Execute(The version number should be cached):
   \ },
   \ ale#fixers#prettier#ApplyFixForVersion(bufnr(''), [])
 
+Execute(Should set --parser to `babylon` by default, < 1.16.0):
+  call ale#test#SetFilename('../prettier-test-files/testfile')
+
+  set filetype=javascript
+
+  AssertEqual
+  \ {
+  \   'command': ale#path#CdString(expand('%:p:h'))
+  \     . ale#Escape(g:ale_javascript_prettier_executable)
+  \     . ' --parser babylon'
+  \     . ' --stdin-filepath %s --stdin',
+  \ },
+  \ ale#fixers#prettier#ApplyFixForVersion(bufnr(''), ['1.6.0'])
+
+Execute(Should set --parser to `babel` by default, >= 1.16.0):
+  call ale#test#SetFilename('../prettier-test-files/testfile')
+
+  set filetype=javascript
+
+  AssertEqual
+  \ {
+  \   'command': ale#path#CdString(expand('%:p:h'))
+  \     . ale#Escape(g:ale_javascript_prettier_executable)
+  \     . ' --parser babel'
+  \     . ' --stdin-filepath %s --stdin',
+  \ },
+  \ ale#fixers#prettier#ApplyFixForVersion(bufnr(''), ['1.16.0'])
+
 Execute(Should set --parser based on filetype, TypeScript):
   call ale#test#SetFilename('../prettier-test-files/testfile')