diff --git a/app/assets/javascripts/diffs/store/diff_file.js b/app/assets/javascripts/diffs/store/diff_file.js
new file mode 100644
index 0000000000000000000000000000000000000000..ca34c9a8e7bffee400b743c06f5c97a3fb8308bf
--- /dev/null
+++ b/app/assets/javascripts/diffs/store/diff_file.js
@@ -0,0 +1,38 @@
+// https://gitlab.com/gitlab-org/frontend/rfcs/-/issues/20
+/* eslint-disable import/prefer-default-export */
+
+const SHA1_REGEX = /^[a-f0-9]{40}$/i;
+// Testers are quick checks to see if we can focus on a single matcher right away, ordered by preference
+const testers = [
+  {
+    type: 'sha1',
+    test: id => SHA1_REGEX.test(id),
+  },
+  {
+    type: 'any',
+    test: () => true,
+  },
+];
+// Matchers are all the ways we know how to identify a Diff File, with `any` as a fallback
+const matchers = {
+  path: function matchesPath(file, path) {
+    return file.file_path === path;
+  },
+  sha1: function matchesSha(file, sha1) {
+    return file.file_hash === sha1;
+  },
+  any: function matchesAnyIdentifier(file, identifier) {
+    return matchers.sha1(file, identifier) || matchers.path(file, identifier);
+  },
+};
+
+function getIdentifierFilter(identifier) {
+  const { type } = testers.find(definition => definition.test(identifier));
+  const check = matchers[type];
+
+  return file => check(file, identifier);
+}
+
+export function getByIdentifier({ identifier, diffFiles }) {
+  return diffFiles.find(getIdentifierFilter(identifier));
+}
diff --git a/spec/frontend/diffs/store/diff_file_spec.js b/spec/frontend/diffs/store/diff_file_spec.js
new file mode 100644
index 0000000000000000000000000000000000000000..4b2f0e6c33cc992e055b8798dee5eb00e2d08b36
--- /dev/null
+++ b/spec/frontend/diffs/store/diff_file_spec.js
@@ -0,0 +1,30 @@
+import { getByIdentifier } from '~/diffs/store/diff_file';
+
+function freeze(thing) {
+  return Object.freeze(thing);
+}
+
+describe('DiffFile utilities', () => {
+  describe('getByIdentifier', () => {
+    const SHA1 = '2c20f4f1829ab02411cf31891e09809acca759f2';
+    const DIFF_FILES = freeze([
+      freeze({ testId: 'alpha', file_hash: SHA1 }),
+      freeze({ testId: 'beta', file_hash: 'file_hash' }),
+      freeze({ testId: 'gamma', file_path: 'file_path' }),
+    ]);
+
+    it.each`
+      identifier                                    | findFile
+      ${'2c20f4f1829ab02411cf31891e09809acca759f2'} | ${'alpha'}
+      ${'file_hash'}                                | ${'beta'}
+      ${'file_path'}                                | ${'gamma'}
+    `(
+      'finds the correct file when given the identifier $identifier',
+      ({ identifier, findFile }) => {
+        const file = getByIdentifier({ diffFiles: DIFF_FILES, identifier });
+
+        expect(file.testId).toEqual(findFile);
+      },
+    );
+  });
+});