util: ignore invalid format specifiers

In util.format, if a percent sign without a known type is encountered,
just print it instead of silently ignoring it and the next character.

PR-URL: https://github.com/nodejs/node/pull/13674
Fixes: https://github.com/nodejs/node/issues/13665
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

lib/util.js

@@ -113,6 +113,12 @@ exports.format = function(f) {
             str += f.slice(lastPos, i);
           str += '%';
           break;
+        default: // any other character is not a correct placeholder
+          if (lastPos < i)
+            str += f.slice(lastPos, i);
+          str += '%';
+          lastPos = i = i + 1;
+          continue;
       }
       lastPos = i = i + 2;
       continue;

test/parallel/test-util-format.js

@@ -126,6 +126,12 @@ assert.strictEqual(util.format('o: %j, a: %j', {}, []), 'o: {}, a: []');
 assert.strictEqual(util.format('o: %j, a: %j', {}), 'o: {}, a: %j');
 assert.strictEqual(util.format('o: %j, a: %j'), 'o: %j, a: %j');
 
+// Invalid format specifiers
+assert.strictEqual(util.format('a% b', 'x'), 'a% b x');
+assert.strictEqual(util.format('percent: %d%, fraction: %d', 10, 0.1),
+                   'percent: 10%, fraction: 0.1');
+assert.strictEqual(util.format('abc%', 1), 'abc% 1');
+
 {
   const o = {};
   o.o = o;