Escape percent signs in formatter output

src/common.js

@@ -98,6 +98,7 @@ function setup(env) {
 				if (typeof formatter === 'function') {
 					const val = args[index];
 					match = formatter.call(self, val);
+					match = String(match).replace(/%/g, '%%');
 
 					// Now we need to remove `args[index]` since it's inlined in the `format`
 					args.splice(index, 1);

test.js

@@ -43,6 +43,19 @@ describe('debug', () => {
 		assert.deepStrictEqual(messages.length, 3);
 	});
 
+	it('escapes percent signs returned by custom formatters', () => {
+		const log = debug('test');
+		log.enabled = true;
+		debug.formatters.x = () => '%j';
+
+		const messages = [];
+		log.log = (...args) => messages.push(args);
+		log('%x', null, 1);
+
+		assert(messages[0][0].includes('%%j'));
+		delete debug.formatters.x;
+	});
+
 	describe('extend namespace', () => {
 		it('should extend namespace', () => {
 			const log = debug('foo');

test.node.js

@@ -36,5 +36,22 @@ describe('debug node', () => {
 			assert.deepStrictEqual(util.formatWithOptions.getCall(0).args[0], options);
 			stdErrWriteStub.restore();
 		});
+
+		it('does not interpret percent signs in formatter output', () => {
+			debug.enable('*');
+			const options = {
+				hideDate: true,
+				colors: false
+			};
+			Object.assign(debug.inspectOpts, options);
+			const stdErrWriteStub = sinon.stub(process.stderr, 'write');
+			const log = debug('percent');
+			log('%o', {'%j': '%j %j %%'}, 1, 2, 3);
+			assert.strictEqual(
+				stdErrWriteStub.firstCall.firstArg,
+				'percent { \'%j\': \'%j %j %%\' } 1 2 3\n'
+			);
+			stdErrWriteStub.restore();
+		});
 	});
 });