diff --git a/format.js b/format.js
index b035911..772a797 100644
--- a/format.js
+++ b/format.js
@@ -97,6 +97,9 @@
           tmp = String(parseFloat(nextArg()).toFixed(precision || 6));
           result += leadingZero ? tmp : tmp.replace(/^0/, '');
           break;
+        case 'j': // JSON
+          result += JSON.stringify(nextArg());
+          break;
         case 'o': // number in octal
           result += '0' + parseInt(nextArg(), 10).toString(8);
           break;
diff --git a/test_format.js b/test_format.js
index b7a7dee..9cd6754 100644
--- a/test_format.js
+++ b/test_format.js
@@ -40,7 +40,9 @@ var tests = [
   [['%.2f', 3.14159], '3.14'],
   [['%0.2f', 3.14159], '3.14'],
   [['%.2f', 0.1234], '.12'],
-  [['%0.2f', 0.1234], '0.12']
+  [['%0.2f', 0.1234], '0.12'],
+  [['foo %j', 42], 'foo 42'],
+  [['foo %j', '42'], 'foo "42"']
 ];
 tests.forEach(function(spec) {
   var args = spec[0];