docs: link to next docs and vice versa

docs/.vitepress/versions.ts

@@ -1,7 +1,25 @@
+import { execSync } from 'child_process';
 import { version } from '../../package.json';
 
-export const currentVersion = `v${version}`;
+const NEXT_TEXT = 'Next';
+
+function readBranchName() {
+  try {
+    return execSync('git branch --show-current').toString('utf8') || NEXT_TEXT;
+  } catch {
+    return NEXT_TEXT;
+  }
+}
+
+const branchName = readBranchName();
+const isNext = !branchName.match(/^v\d+$/);
+
+export const currentVersion = isNext ? NEXT_TEXT : `v${version}`;
+
+const nextVersion = { version: NEXT_TEXT, link: 'https://next.fakerjs.dev/' };
+const latestVersion = { version: `v${version}`, link: 'https://fakerjs.dev/' };
 
 export const oldVersions = [
+  isNext ? latestVersion : nextVersion,
   { version: 'v6.3.1', link: 'https://v6.fakerjs.dev/' },
 ];

test/docs/versions.spec.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from 'vitest';
+import { currentVersion, oldVersions } from '../../docs/.vitepress/versions';
+import { version } from '../../package.json';
+
+function extractMajorVersionNumber(version: string): number {
+  return Number(version.split('.')[0].substring(1));
+}
+
+describe('docs versions', () => {
+  it('should have a complete set of oldVersions', () => {
+    const versionText = `v${version}`;
+
+    expect(oldVersions.length).toBeGreaterThanOrEqual(1);
+    const currentMajorVersion = extractMajorVersionNumber(versionText);
+
+    if (currentVersion === 'Next') {
+      expect(oldVersions[0]).toEqual({
+        version: versionText,
+        link: 'https://fakerjs.dev/',
+      });
+
+      for (let i = 0; i < oldVersions.length; i++) {
+        const oldMajorVersion = extractMajorVersionNumber(
+          oldVersions[i].version
+        );
+        expect(oldMajorVersion).toBe(currentMajorVersion - i);
+      }
+    } else {
+      expect(oldVersions[0]).toEqual({
+        version: 'Next',
+        link: 'https://next.fakerjs.dev/',
+      });
+
+      for (let i = 1; i < oldVersions.length; i++) {
+        const oldMajorVersion = extractMajorVersionNumber(
+          oldVersions[i].version
+        );
+        expect(oldMajorVersion).toBe(currentMajorVersion - i);
+      }
+    }
+  });
+});