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. Fixes: nodejs#13665
targos · Jun 14, 2017 · 635e2a8 · 635e2a8
1 parent 448c4c6
commit 635e2a8
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diff --git a/lib/util.js b/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;

diff --git a/test/parallel/test-util-format.js b/test/parallel/test-util-format.js
@@ -126,6 +126,11 @@ 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');
+
 {
   const o = {};
   o.o = o;