Collection editor: Format topic as hashtag (#38153)

app/javascript/mastodon/utils/hashtags.test.ts

@@ -0,0 +1,28 @@
+import { inputToHashtag } from './hashtags';
+
+describe('inputToHashtag', () => {
+  test.concurrent.each([
+    ['', ''],
+    // Prepend or keep hashtag
+    ['mastodon', '#mastodon'],
+    ['#mastodon', '#mastodon'],
+    // Preserve trailing whitespace
+    ['mastodon  ', '#mastodon  '],
+    ['   ', '#   '],
+    // Collapse whitespace & capitalise first character
+    ['cats of mastodon', '#catsOfMastodon'],
+    ['x y z', '#xYZ'],
+    ['   mastodon', '#mastodon'],
+    // Preserve initial casing
+    ['Log in', '#LogIn'],
+    ['#NaturePhotography', '#NaturePhotography'],
+    // Normalise hash symbol variant
+    ['＃nature', '#nature'],
+    ['＃Nature Photography', '#NaturePhotography'],
+    // Allow special characters
+    ['hello-world', '#hello-world'],
+    ['hello,world', '#hello,world'],
+  ])('for input "%s", return "%s"', (input, expected) => {
+    expect(inputToHashtag(input)).toBe(expected);
+  });
+});

app/javascript/mastodon/utils/hashtags.ts

@@ -27,3 +27,35 @@ const buildHashtagRegex = () => {
 export const HASHTAG_PATTERN_REGEX = buildHashtagPatternRegex();
 
 export const HASHTAG_REGEX = buildHashtagRegex();
+
+/**
+ * Formats an input string as a hashtag:
+ * - Prepends `#` unless present
+ * - Strips spaces (except at the end, to allow typing it)
+ * - Capitalises first character after stripped space
+ */
+export const inputToHashtag = (input: string): string => {
+  if (!input) {
+    return '';
+  }
+
+  const trailingSpace = /\s+$/.exec(input)?.[0] ?? '';
+  const trimmedInput = input.trimEnd();
+
+  const withoutHash =
+    trimmedInput.startsWith('#') || trimmedInput.startsWith('＃')
+      ? trimmedInput.slice(1)
+      : trimmedInput;
+
+  // Split by space, filter empty strings, and capitalise the start of each word but the first
+  const words = withoutHash
+    .split(/\s+/)
+    .filter((word) => word.length > 0)
+    .map((word, index) =>
+      index === 0
+        ? word
+        : word.charAt(0).toUpperCase() + word.slice(1).toLowerCase(),
+    );
+
+  return `#${words.join('')}${trailingSpace}`;
+};