feat: add concat-css-classes helper method

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/utils/concat-css-classes.spec.ts

@@ -0,0 +1,32 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { concatCssClasses } from './concat-css-classes'
+
+describe('concat css classes', () => {
+  it('works with a map', () => {
+    expect(concatCssClasses({ a: true, b: false, c: true })).toBe('a c')
+  })
+
+  it('works with a string array ', () => {
+    expect(concatCssClasses('a', 'b', 'c')).toBe('a b c')
+  })
+
+  it('works with a string array and map', () => {
+    expect(concatCssClasses('a', 'b', 'c', { d: true, e: false, f: true })).toBe('a b c d f')
+  })
+
+  it("doesn't include undefined and null", () => {
+    expect(concatCssClasses(undefined, null)).toBe('')
+  })
+
+  it("doesn't include duplicates", () => {
+    expect(concatCssClasses('a', 'a', { a: true })).toBe('a')
+  })
+
+  it("doesn't include empty class names", () => {
+    expect(concatCssClasses('a', '', { '': true })).toBe('a')
+  })
+})