From b80fc0f096400c22fe1c447ed6305f83e8a19b11 Mon Sep 17 00:00:00 2001
From: JSap0914 <JSap0914@users.noreply.github.com>
Date: Tue, 23 Jun 2026 21:23:36 +0900
Subject: [PATCH] fix: enabled returns undefined instead of false when DEBUG is
 not set

When enable() is called with a non-string argument (including undefined,
which happens at startup when process.env.DEBUG is absent), it assigns
createDebug.namespaces = undefined.  The per-instance enabled getter
caches results using namespacesCache, which is initialised to undefined
in the closure.  The cache-miss guard

  if (namespacesCache !== createDebug.namespaces)

therefore evaluates to false on the very first access (undefined !==
undefined is false), so createDebug.enabled(namespace) is never invoked,
and enabledCache stays undefined.  Every subsequent read of instance.enabled
returns undefined instead of false until a second, string-valued enable()
call is made.

Fix: normalise createDebug.namespaces to an empty string whenever the
argument is not a string, consistent with how the split step already
treats non-string input.
---
 src/common.js |  2 +-
 test.js       | 31 ++++++++++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/common.js b/src/common.js
index 141cb578..a3ae3b24 100644
--- a/src/common.js
+++ b/src/common.js
@@ -161,7 +161,7 @@ function setup(env) {
 	*/
 	function enable(namespaces) {
 		createDebug.save(namespaces);
-		createDebug.namespaces = namespaces;
+		createDebug.namespaces = typeof namespaces === 'string' ? namespaces : '';
 
 		createDebug.names = [];
 		createDebug.skips = [];
diff --git a/test.js b/test.js
index a1d6f633..83f2ebc3 100644
--- a/test.js
+++ b/test.js
@@ -137,4 +137,33 @@ describe('debug', () => {
 			assert.deepStrictEqual(messages, ['test2', 'test3']);
 		});
 	});
-});
+
+	describe('enabled before any explicit enable() call', () => {
+		it('returns false (not undefined) when DEBUG env var is absent', () => {
+			// Regression: when enable() is called with undefined (e.g. DEBUG is
+			// not set), createDebug.namespaces is set to undefined.  The per-instance
+			// cache initialises namespacesCache to undefined too, so the cache
+			// comparison `namespacesCache !== createDebug.namespaces` evaluates to
+			// false and enabled() is never invoked, leaving enabledCache as
+			// undefined rather than false.
+			const savedDebug = process.env.DEBUG;
+			delete process.env.DEBUG;
+			// Re-create a fresh debug factory with no DEBUG env var
+			Object.keys(require.cache).forEach(k => {
+				if (k.includes('debug/src')) {
+					delete require.cache[k];
+				}
+			});
+			const freshDebug = require('./src');
+			const log = freshDebug('test:namespace');
+			assert.strictEqual(typeof log.enabled, 'boolean',
+				'enabled should be a boolean, not ' + typeof log.enabled);
+			assert.strictEqual(log.enabled, false,
+				'enabled should be false when no namespaces are active');
+			// Restore
+			if (savedDebug !== undefined) {
+				process.env.DEBUG = savedDebug;
+			}
+		});
+	});
+});
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