Keep spinner frames out of redirected output

src/__tests__/utils/spinner.test.ts

@@ -0,0 +1,90 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { createSpinner } from '../../utils/spinner';
+
+describe('spinner', () => {
+  let stderrWriteSpy: ReturnType<typeof vi.spyOn>;
+  let originalIsTTY: boolean | undefined;
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+    originalIsTTY = process.stderr.isTTY;
+    stderrWriteSpy = vi
+      .spyOn(process.stderr, 'write')
+      .mockImplementation(() => true);
+  });
+
+  afterEach(() => {
+    Object.defineProperty(process.stderr, 'isTTY', {
+      configurable: true,
+      value: originalIsTTY,
+    });
+    vi.useRealTimers();
+    vi.restoreAllMocks();
+  });
+
+  function setInteractive(isInteractive: boolean): void {
+    Object.defineProperty(process.stderr, 'isTTY', {
+      configurable: true,
+      value: isInteractive,
+    });
+  }
+
+  it('animates when stderr is attached to a terminal', () => {
+    setInteractive(true);
+    const spinner = createSpinner('Working...');
+
+    spinner.start();
+
+    expect(stderrWriteSpy).toHaveBeenNthCalledWith(1, '\r\x1b[K');
+    expect(stderrWriteSpy).toHaveBeenNthCalledWith(2, '⠋ Working...');
+
+    vi.advanceTimersByTime(80);
+
+    expect(stderrWriteSpy).toHaveBeenNthCalledWith(3, '\r\x1b[K');
+    expect(stderrWriteSpy).toHaveBeenNthCalledWith(4, '⠙ Working...');
+
+    spinner.succeed('Done');
+
+    expect(stderrWriteSpy).toHaveBeenNthCalledWith(5, '\r\x1b[K');
+    expect(stderrWriteSpy).toHaveBeenNthCalledWith(6, '✓ Done\n');
+
+    vi.advanceTimersByTime(200);
+    expect(stderrWriteSpy).toHaveBeenCalledTimes(6);
+  });
+
+  it('writes only the final status when stderr is redirected', () => {
+    setInteractive(false);
+    const spinner = createSpinner('Working...');
+
+    spinner.start();
+    spinner.update('Still working...');
+    vi.advanceTimersByTime(240);
+    spinner.succeed('Done');
+
+    expect(stderrWriteSpy).toHaveBeenCalledTimes(1);
+    expect(stderrWriteSpy).toHaveBeenCalledWith('✓ Done\n');
+  });
+
+  it('keeps failure messages in redirected output', () => {
+    setInteractive(false);
+    const spinner = createSpinner('Working...');
+
+    spinner.start();
+    spinner.fail('Request failed');
+
+    expect(stderrWriteSpy).toHaveBeenCalledTimes(1);
+    expect(stderrWriteSpy).toHaveBeenCalledWith('✗ Request failed\n');
+  });
+
+  it('uses the latest message as the redirected final status', () => {
+    setInteractive(false);
+    const spinner = createSpinner('Working...');
+
+    spinner.start();
+    spinner.update('Still working...');
+    spinner.succeed();
+
+    expect(stderrWriteSpy).toHaveBeenCalledTimes(1);
+    expect(stderrWriteSpy).toHaveBeenCalledWith('✓ Still working...\n');
+  });
+});

src/utils/spinner.ts

@@ -16,6 +16,7 @@ export function createSpinner(initialMessage: string = ''): Spinner {
   let frameIndex = 0;
   let interval: ReturnType<typeof setInterval> | null = null;
   let currentMessage = initialMessage;
+  const isInteractive = process.stderr.isTTY === true;
 
   const clearLine = () => {
     process.stderr.write('\r\x1b[K');
@@ -31,7 +32,7 @@ export function createSpinner(initialMessage: string = ''): Spinner {
   return {
     start(message?: string) {
       if (message) currentMessage = message;
-      if (interval) return;
+      if (!isInteractive || interval) return;
       render();
       interval = setInterval(render, 80);
     },
@@ -45,7 +46,9 @@ export function createSpinner(initialMessage: string = ''): Spinner {
         clearInterval(interval);
         interval = null;
       }
-      clearLine();
+      if (isInteractive) {
+        clearLine();
+      }
       if (finalMessage) {
         process.stderr.write(`${finalMessage}\n`);
       }