Move all functions for fixing things to autoload/ale/fixers, and only accept the lines of input where needed.

autoload/ale/fixers/prettier.vim

@@ -0,0 +1,25 @@
+" Author: tunnckoCore (Charlike Mike Reagent) <mameto2011@gmail.com>,
+"         w0rp <devw0rp@gmail.com>
+" Description: Integration of Prettier with ALE.
+
+call ale#Set('javascript_prettier_executable', 'prettier')
+call ale#Set('javascript_prettier_use_global', 0)
+
+function! ale#handlers#prettier#GetExecutable(buffer) abort
+    return ale#node#FindExecutable(a:buffer, 'javascript_prettier', [
+    \   'node_modules/prettier-cli/index.js',
+    \   'node_modules/.bin/prettier',
+    \])
+endfunction
+
+function! ale#handlers#prettier#Fix(buffer, lines) abort
+    let l:options = ale#Var(a:buffer, 'javascript_prettier_options')
+
+    return {
+    \   'command': ale#Escape(ale#handlers#prettier#GetExecutable(a:buffer))
+    \       . ' %t'
+    \       . ' ' . l:options
+    \       . ' --write',
+    \   'read_temporary_file': 1,
+    \}
+endfunction