Add utility to determine if a file is considered diffable

app/assets/javascripts/diffs/utils/diff_file.js

+import { diffViewerModes as viewerModes } from '~/ide/constants';
 import { truncateSha } from '~/lib/utils/text_utility';
 import { uuids } from '~/lib/utils/uuids';
 
@@ -46,6 +47,8 @@ function identifier(file) {
   })[0];
 }
 
+export const isNotDiffable = (file) => file?.viewer?.name === viewerModes.not_diffable;
+
 export function prepareRawDiffFile({ file, allFiles, meta = false }) {
   const additionalProperties = {
     brokenSymlink: fileSymlinkInformation(file, allFiles),

spec/frontend/diffs/utils/diff_file_spec.js

-import { prepareRawDiffFile, getShortShaFromFile } from '~/diffs/utils/diff_file';
+import { prepareRawDiffFile, getShortShaFromFile, isNotDiffable } from '~/diffs/utils/diff_file';
+import { diffViewerModes } from '~/ide/constants';
 
 function getDiffFiles() {
   const loadFull = 'namespace/project/-/merge_requests/12345/diff_for_path?file_identifier=abc';
@@ -154,4 +155,26 @@ describe('diff_file utilities', () => {
       expect(getShortShaFromFile({ content_sha: cs })).toBe(response);
     });
   });
+
+  describe('isNotDiffable', () => {
+    it.each`
+      bool     | vw
+      ${true}  | ${diffViewerModes.not_diffable}
+      ${false} | ${diffViewerModes.text}
+      ${false} | ${diffViewerModes.image}
+    `('returns $bool when the viewer is $vw', ({ bool, vw }) => {
+      expect(isNotDiffable({ viewer: { name: vw } })).toBe(bool);
+    });
+
+    it.each`
+      file
+      ${undefined}
+      ${null}
+      ${{}}
+      ${{ viewer: undefined }}
+      ${{ viewer: null }}
+    `('reports `false` when the file is `$file`', ({ file }) => {
+      expect(isNotDiffable(file)).toBe(false);
+    });
+  });
 });