Refactor `readmeFile`

app/assets/javascripts/repository/utils/readme.js

-const MARKDOWN_EXTENSIONS = ['mdown', 'mkd', 'mkdn', 'md', 'markdown'];
-const ASCIIDOC_EXTENSIONS = ['adoc', 'ad', 'asciidoc'];
-const OTHER_EXTENSIONS = ['textile', 'rdoc', 'org', 'creole', 'wiki', 'mediawiki', 'rst'];
-const EXTENSIONS = [...MARKDOWN_EXTENSIONS, ...ASCIIDOC_EXTENSIONS, ...OTHER_EXTENSIONS];
-const PLAIN_FILENAMES = ['readme', 'index'];
-const FILE_REGEXP = new RegExp(
-  `^(${PLAIN_FILENAMES.join('|')})(.(${EXTENSIONS.join('|')}))?$`,
-  'i',
-);
-const PLAIN_FILE_REGEXP = new RegExp(`^(${PLAIN_FILENAMES.join('|')})`, 'i');
-const EXTENSIONS_REGEXP = new RegExp(`.(${EXTENSIONS.join('|')})$`, 'i');
+const FILENAMES = ['index', 'readme'];
 
-// eslint-disable-next-line import/prefer-default-export
-export const readmeFile = blobs => {
-  const readMeFiles = blobs.filter(f => f.name.search(FILE_REGEXP) !== -1);
+const MARKUP_EXTENSIONS = [
+  'ad',
+  'adoc',
+  'asciidoc',
+  'creole',
+  'markdown',
+  'md',
+  'mdown',
+  'mediawiki',
+  'mkd',
+  'mkdn',
+  'org',
+  'rdoc',
+  'rst',
+  'textile',
+  'wiki',
+];
 
-  const previewableReadme = readMeFiles.find(f => f.name.search(EXTENSIONS_REGEXP) !== -1);
-  const plainReadme = readMeFiles.find(f => f.name.search(PLAIN_FILE_REGEXP) !== -1);
+const isRichReadme = file => {
+  const re = new RegExp(`^(${FILENAMES.join('|')})\\.(${MARKUP_EXTENSIONS.join('|')})$`, 'i');
+  return re.test(file.name);
+};
 
-  return previewableReadme || plainReadme;
+const isPlainReadme = file => {
+  const re = new RegExp(`^(${FILENAMES.join('|')})$`, 'i');
+  return re.test(file.name);
 };
+
+// eslint-disable-next-line import/prefer-default-export
+export const readmeFile = blobs => blobs.find(isRichReadme) || blobs.find(isPlainReadme);

spec/frontend/repository/utils/readme_spec.js

 import { readmeFile } from '~/repository/utils/readme';
 
 describe('readmeFile', () => {
-  describe('markdown files', () => {
-    it('returns markdown file', () => {
-      expect(readmeFile([{ name: 'README' }, { name: 'README.md' }])).toEqual({
-        name: 'README.md',
-      });
+  it('prefers README with markup over plain text README', () => {
+    expect(readmeFile([{ name: 'README' }, { name: 'README.md' }])).toEqual({
+      name: 'README.md',
+    });
+  });
 
-      expect(readmeFile([{ name: 'README' }, { name: 'index.md' }])).toEqual({
-        name: 'index.md',
-      });
+  it('is case insensitive', () => {
+    expect(readmeFile([{ name: 'README' }, { name: 'readme.rdoc' }])).toEqual({
+      name: 'readme.rdoc',
     });
   });
 
-  describe('plain files', () => {
-    it('returns plain file', () => {
-      expect(readmeFile([{ name: 'README' }, { name: 'TEST.md' }])).toEqual({
-        name: 'README',
-      });
+  it('returns the first README found', () => {
+    expect(readmeFile([{ name: 'INDEX.adoc' }, { name: 'README.md' }])).toEqual({
+      name: 'INDEX.adoc',
+    });
+  });
 
-      expect(readmeFile([{ name: 'readme' }, { name: 'TEST.md' }])).toEqual({
-        name: 'readme',
-      });
+  it('expects extension to be separated by dot', () => {
+    expect(readmeFile([{ name: 'readmeXorg' }, { name: 'index.org' }])).toEqual({
+      name: 'index.org',
     });
   });
 
-  describe('non-previewable file', () => {
-    it('returns undefined', () => {
-      expect(readmeFile([{ name: 'index.js' }, { name: 'TEST.md' }])).toBe(undefined);
+  it('returns plain text README when there is no README with markup', () => {
+    expect(readmeFile([{ name: 'README' }, { name: 'NOT_README.md' }])).toEqual({
+      name: 'README',
     });
   });
+
+  it('returns undefined when there are no appropriate files', () => {
+    expect(readmeFile([{ name: 'index.js' }, { name: 'md.README' }])).toBe(undefined);
+    expect(readmeFile([])).toBe(undefined);
+  });
 });