From f9b6603134f3cb7760e03616b2167839037649d6 Mon Sep 17 00:00:00 2001
From: Yunare Maia <yunare@gmail.com>
Date: Thu, 3 Sep 2026 20:16:22 +0000
Subject: [PATCH] fix: preserve existing namespaces on enable() call

Previously, calling `enable()` would reset the `names` and `skips`
arrays, clearing any namespaces that were previously enabled.

This caused behavior where `debug.enable('bar')` after `DEBUG=foo`
would silently disable the 'foo' namespace, surprising users who
expected both to be active.

The fix makes `enable()` merge namespaces instead of replacing
them. Empty/null/undefined still resets (for `disable()` compatibility).

Fixes debug-js/debug#425
---
 src/common.js | 20 ++++++++++++++++----
 test.js       | 12 ++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/common.js b/src/common.js
index 141cb578..f5f72a22 100644
--- a/src/common.js
+++ b/src/common.js
@@ -160,12 +160,22 @@ function setup(env) {
 	* @api public
 	*/
 	function enable(namespaces) {
+		// Normalize: empty/null/undefined means "disable all" (full replace with empty).
+		// Non-empty string means "merge" with existing namespaces.
+		// See: https://github.com/debug-js/debug/issues/425
+		const isReset = !namespaces || (typeof namespaces === 'string' && namespaces.trim() === '');
+
 		createDebug.save(namespaces);
 		createDebug.namespaces = namespaces;
 
-		createDebug.names = [];
-		createDebug.skips = [];
+		if (isReset) {
+			createDebug.names = [];
+			createDebug.skips = [];
+			return;
+		}
 
+		// Merge: preserve previously enabled namespaces when enable() is called
+		// again (e.g., `debug.enable('bar')` after `DEBUG=foo` won't clear 'foo').
 		const split = (typeof namespaces === 'string' ? namespaces : '')
 			.trim()
 			.replace(/\s+/g, ',')
@@ -174,8 +184,10 @@ function setup(env) {
 
 		for (const ns of split) {
 			if (ns[0] === '-') {
-				createDebug.skips.push(ns.slice(1));
-			} else {
+				if (!createDebug.skips.includes(ns.slice(1))) {
+					createDebug.skips.push(ns.slice(1));
+				}
+			} else if (!createDebug.names.includes(ns)) {
 				createDebug.names.push(ns);
 			}
 		}
diff --git a/test.js b/test.js
index a1d6f633..89129a1c 100644
--- a/test.js
+++ b/test.js
@@ -27,6 +27,12 @@ describe('debug', () => {
 		debug.enable('test:12345');
 		assert.deepStrictEqual(debug('test:12345').enabled, true);
 		assert.deepStrictEqual(debug('test:67890').enabled, false);
+
+		// Enable() should merge, not replace (issue #425)
+		debug.enable('test:67890');
+		assert.deepStrictEqual(debug('test:12345').enabled, true);
+		assert.deepStrictEqual(debug('test:67890').enabled, true);
+		debug.disable();
 	});
 
 	it('uses custom log function', () => {
@@ -82,12 +88,14 @@ describe('debug', () => {
 
 	describe('rebuild namespaces string (disable)', () => {
 		it('handle names, skips, and wildcards', () => {
+			debug.disable();
 			debug.enable('test,abc*,-abc');
 			const namespaces = debug.disable();
 			assert.deepStrictEqual(namespaces, 'test,abc*,-abc');
 		});
 
 		it('handles empty', () => {
+			debug.disable();
 			debug.enable('');
 			const namespaces = debug.disable();
 			assert.deepStrictEqual(namespaces, '');
@@ -96,18 +104,21 @@ describe('debug', () => {
 		});
 
 		it('handles all', () => {
+			debug.disable();
 			debug.enable('*');
 			const namespaces = debug.disable();
 			assert.deepStrictEqual(namespaces, '*');
 		});
 
 		it('handles skip all', () => {
+			debug.disable();
 			debug.enable('-*');
 			const namespaces = debug.disable();
 			assert.deepStrictEqual(namespaces, '-*');
 		});
 
 		it('names+skips same with new string', () => {
+			debug.disable();
 			debug.enable('test,abc*,-abc');
 			const oldNames = [...debug.names];
 			const oldSkips = [...debug.skips];
@@ -119,6 +130,7 @@ describe('debug', () => {
 		});
 
 		it('handles re-enabling existing instances', () => {
+			debug.disable();
 			debug.disable('*');
 			const inst = debug('foo');
 			const messages = [];