feat: Return namespaces string when invoking disable()

Closes debug-js#523

feat: Add unit tests for disable return value

fix: Correct spelling in test case description

src/common.js

@@ -187,10 +187,16 @@ function setup(env) {
 	/**
 	* Disable debug output.
 	*
+	* @return {String} namespaces
 	* @api public
 	*/
 	function disable() {
+		const namespaces = [
+			...createDebug.names.map(toNamespace),
+			...createDebug.skips.map(toNamespace).map(namespace => '-' + namespace)
+		].join(',');
 		createDebug.enable('');
+		return namespaces;
 	}
 
 	/**
@@ -223,6 +229,19 @@ function setup(env) {
 		return false;
 	}
 
+	/**
+	* Convert regexp to namespace
+	*
+	* @param {RegExp} regxep
+	* @return {String} namespace
+	* @api private
+	*/
+	function toNamespace(regexp) {
+		return regexp.toString()
+			.substring(2, regexp.toString().length - 2)
+			.replace(/\.\*\?$/, '*');
+	}
+
 	/**
 	* Coerce `val`.
 	*

test.js

@@ -81,4 +81,30 @@ describe('debug', () => {
 			expect(logBar.namespace).to.be.equal('foobar');
 		});
 	});
+
+	describe('rebuild namespaces string (disable)', () => {
+		it('handle names, skips, and wildcards', () => {
+			debug.enable('test,abc*,-abc');
+			const namespaces = debug.disable();
+			expect(namespaces).to.equal('test,abc*,-abc');
+		});
+
+		it('handles empty', () => {
+			debug.enable('');
+			const namespaces = debug.disable();
+			expect(namespaces).to.equal('');
+		});
+
+		it('handles all', () => {
+			debug.enable('*');
+			const namespaces = debug.disable();
+			expect(namespaces).to.equal('*');
+		});
+
+		it('handles skip all', () => {
+			debug.enable('-*');
+			const namespaces = debug.disable();
+			expect(namespaces).to.equal('-*');
+		});
+	});
 });