fix: add file extension util (#488)

* fix: add file extension util

* chore: add link to SO post

src/item/fileItem/fileItem.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from 'vitest';
+
+import { getFileExtension } from './fileItem.js';
+
+describe('File Item utils', () => {
+  describe('getFileExtension', () => {
+    it('Simple extensions', () => {
+      expect(getFileExtension('sample.txt')).toEqual('.txt');
+      expect(getFileExtension('sample.jpeg')).toEqual('.jpeg');
+      expect(getFileExtension('s.t')).toEqual('.t');
+    });
+    it('Simple extensions without dots', () => {
+      expect(getFileExtension('sample.txt', { includeDot: false })).toEqual(
+        'txt',
+      );
+      expect(
+        getFileExtension('sample.test.txt', { includeDot: false }),
+      ).toEqual('txt');
+    });
+    it('Filename with multiple extensions', () => {
+      expect(getFileExtension('sample.test.txt')).toEqual('.txt');
+      expect(getFileExtension('sample.test.hello.wow.txt')).toEqual('.txt');
+    });
+    it('No extensions return undefined', () => {
+      expect(getFileExtension('sample')).toEqual(undefined);
+      expect(getFileExtension('sample.')).toEqual(undefined);
+    });
+  });
+});

src/item/fileItem/fileItem.ts

@@ -52,6 +52,27 @@ export const buildS3FileExtra = (
   [ItemType.S3_FILE]: extra,
 });
 
+/**
+ * Extract extension from filename.
+ * @param fileName Name of the file to get the extension from
+ * @param options Options object, currently contains `includeDot` to return the extension with a leading dot, for easy concatenation, this option is true by default
+ * @returns
+ */
+export const getFileExtension = (
+  fileName: string,
+  { includeDot = true } = {},
+): string | undefined => {
+  // this code has been adapted from https://stackoverflow.com/a/680982
+  const extensionRegex = /(?:\.([^.]+))?$/;
+  const extension = extensionRegex.exec(fileName)?.[1];
+  if (extension) {
+    if (includeDot) {
+      return `.${extension}`;
+    }
+    return extension;
+  }
+};
+
 /**
  * File Settings
  */