fix: merge namespaces in enable() instead of flushing them

README.md

@@ -313,11 +313,19 @@ Usage :
 `enable(namespaces)`  
 `namespaces` can include modes separated by a colon and wildcards.
 
-Note that calling `enable()` completely overrides previously set DEBUG variable : 
+Calling `enable()` merges with previously set namespaces (including `DEBUG`)
+instead of replacing them:
 
 ```
-$ DEBUG=foo node -e 'var dbg = require("debug"); dbg.enable("bar"); console.log(dbg.enabled("foo"))'
-=> false
+$ DEBUG=foo node -e 'var dbg = require("debug"); dbg.enable("bar"); console.log(dbg.enabled("foo"), dbg.enabled("bar"))'
+=> true true
+```
+
+To replace the set entirely, disable first:
+
+```js
+debug.disable();
+debug.enable('bar');
 ```
 
 `disable()`

src/common.js

@@ -153,32 +153,64 @@ function setup(env) {
 	}
 
 	/**
-	* Enables a debug mode by namespaces. This can include modes
-	* separated by a colon and wildcards.
+	* Parse a namespace list into individual tokens.
 	*
 	* @param {String} namespaces
-	* @api public
+	* @return {Array}
+	* @api private
 	*/
-	function enable(namespaces) {
-		createDebug.save(namespaces);
-		createDebug.namespaces = namespaces;
-
-		createDebug.names = [];
-		createDebug.skips = [];
-
-		const split = (typeof namespaces === 'string' ? namespaces : '')
+	function parseNamespaces(namespaces) {
+		return (typeof namespaces === 'string' ? namespaces : '')
 			.trim()
 			.replace(/\s+/g, ',')
 			.split(',')
 			.filter(Boolean);
+	}
+
+	/**
+	* Serialize the current names/skips into a namespace string.
+	*
+	* @return {String}
+	* @api private
+	*/
+	function formatNamespaces() {
+		return [
+			...createDebug.names,
+			...createDebug.skips.map(namespace => '-' + namespace)
+		].join(',');
+	}
+
+	/**
+	* Enables a debug mode by namespaces. This can include modes
+	* separated by a colon and wildcards.
+	*
+	* New namespaces are merged with those already enabled.
+	* Call `disable()` first to replace the set instead of extending it.
+	*
+	* @param {String} namespaces
+	* @api public
+	*/
+	function enable(namespaces) {
+		const split = parseNamespaces(namespaces);
 
 		for (const ns of split) {
 			if (ns[0] === '-') {
-				createDebug.skips.push(ns.slice(1));
+				const name = ns.slice(1);
+				createDebug.names = createDebug.names.filter(existing => existing !== name);
+				if (!createDebug.skips.includes(name)) {
+					createDebug.skips.push(name);
+				}
 			} else {
-				createDebug.names.push(ns);
+				createDebug.skips = createDebug.skips.filter(existing => existing !== ns);
+				if (!createDebug.names.includes(ns)) {
+					createDebug.names.push(ns);
+				}
 			}
 		}
+
+		const merged = formatNamespaces();
+		createDebug.save(merged);
+		createDebug.namespaces = merged;
 	}
 
 	/**
@@ -231,11 +263,11 @@ function setup(env) {
 	* @api public
 	*/
 	function disable() {
-		const namespaces = [
-			...createDebug.names,
-			...createDebug.skips.map(namespace => '-' + namespace)
-		].join(',');
-		createDebug.enable('');
+		const namespaces = formatNamespaces();
+		createDebug.names = [];
+		createDebug.skips = [];
+		createDebug.save('');
+		createDebug.namespaces = '';
 		return namespaces;
 	}
 

test.js

@@ -4,6 +4,10 @@ const assert = require('assert');
 const debug = require('./src');
 
 describe('debug', () => {
+	beforeEach(() => {
+		debug.disable();
+	});
+
 	it('passes a basic sanity check', () => {
 		const log = debug('test');
 		log.enabled = true;
@@ -29,6 +33,68 @@ describe('debug', () => {
 		assert.deepStrictEqual(debug('test:67890').enabled, false);
 	});
 
+	describe('enable() does not flush existing namespaces (#425)', () => {
+		it('merges subsequent enable() calls with previously enabled namespaces', () => {
+			debug.enable('foo');
+			debug.enable('bar');
+
+			assert.deepStrictEqual(debug.enabled('foo'), true);
+			assert.deepStrictEqual(debug.enabled('bar'), true);
+		});
+
+		it('preserves namespaces from a prior enable() like DEBUG=foo then enable("bar")', () => {
+			debug.enable('foo');
+			debug.enable('bar');
+
+			assert.deepStrictEqual(debug.enabled('foo'), true);
+			assert.deepStrictEqual(debug.enabled('bar'), true);
+			assert.deepStrictEqual(debug.disable(), 'foo,bar');
+		});
+
+		it('does not drop earlier namespaces when a later module enables its own', () => {
+			debug.enable('my-module');
+			debug.enable('my-dep-module');
+
+			assert.deepStrictEqual(debug.enabled('my-module'), true);
+			assert.deepStrictEqual(debug.enabled('my-dep-module'), true);
+		});
+
+		it('can still replace the set via disable() then enable()', () => {
+			debug.enable('foo');
+			debug.disable();
+			debug.enable('bar');
+
+			assert.deepStrictEqual(debug.enabled('foo'), false);
+			assert.deepStrictEqual(debug.enabled('bar'), true);
+		});
+
+		it('adds skip patterns without dropping other namespaces', () => {
+			debug.enable('foo');
+			debug.enable('-foo');
+			debug.enable('bar');
+
+			assert.deepStrictEqual(debug.enabled('foo'), false);
+			assert.deepStrictEqual(debug.enabled('bar'), true);
+		});
+
+		it('re-enabling a skipped namespace removes the skip', () => {
+			debug.enable('foo');
+			debug.enable('-foo');
+			assert.deepStrictEqual(debug.enabled('foo'), false);
+
+			debug.enable('foo');
+			assert.deepStrictEqual(debug.enabled('foo'), true);
+		});
+
+		it('does not duplicate namespaces when enable() is repeated', () => {
+			debug.enable('foo');
+			debug.enable('foo');
+			debug.enable('bar,foo');
+
+			assert.deepStrictEqual(debug.disable(), 'foo,bar');
+		});
+	});
+
 	it('uses custom log function', () => {
 		const log = debug('test');
 		log.enabled = true;