--- a/js/src/jsapi-tests/moz.build
+++ b/js/src/jsapi-tests/moz.build
@@ -68,16 +68,17 @@ UNIFIED_SOURCES += [
     'testNewObject.cpp',
     'testNewTargetInvokeConstructor.cpp',
     'testNullRoot.cpp',
     'testObjectEmulatingUndefined.cpp',
     'testOOM.cpp',
     'testParseJSON.cpp',
     'testPersistentRooted.cpp',
     'testPreserveJitCode.cpp',
+    'testPrintf.cpp',
     'testPrivateGCThingValue.cpp',
     'testProfileStrings.cpp',
     'testPropCache.cpp',
     'testRegExp.cpp',
     'testResolveRecursion.cpp',
     'tests.cpp',
     'testSameValue.cpp',
     'testSavedStacks.cpp',
@@ -141,16 +142,16 @@ if CONFIG['ENABLE_INTL_API'] and CONFIG[
 
 USE_LIBS += [
     'static:js',
 ]
 
 OS_LIBS += CONFIG['MOZ_ZLIB_LIBS']
 
 if CONFIG['GNU_CXX']:
-    CXXFLAGS += ['-Wno-shadow']
+    CXXFLAGS += ['-Wno-shadow', '-Werror=format']
 
 # This is intended as a temporary workaround to enable VS2015.
 if CONFIG['_MSC_VER']:
     CXXFLAGS += ['-wd4312']
 
 DEFINES['topsrcdir'] = '%s/js/src' % TOPSRCDIR
 OBJDIR_PP_FILES.js.src['jsapi-tests'] += ['jsapi-tests-gdb.py.in']

new file mode 100644
--- /dev/null
+++ b/js/src/jsapi-tests/testPrintf.cpp
@@ -0,0 +1,71 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/SizePrintfMacros.h"
+
+#include <stdarg.h>
+
+#include "jsprf.h"
+
+#include "jsapi-tests/tests.h"
+
+static bool
+MOZ_FORMAT_PRINTF(2, 3)
+print_one (const char *expect, const char *fmt, ...)
+{
+    va_list ap;
+
+    va_start(ap, fmt);
+    char *output = JS_vsmprintf (fmt, ap);
+    va_end(ap);
+
+    bool result = output && !strcmp(output, expect);
+    JS_smprintf_free(output);
+
+    return result;
+}
+
+static const char *
+zero()
+{
+    return nullptr;
+}
+
+BEGIN_TEST(testPrintf)
+{
+    CHECK(print_one("23", "%d", 23));
+    CHECK(print_one("-1", "%d", -1));
+    CHECK(print_one("23", "%u", 23u));
+    CHECK(print_one("0x17", "0x%x", 23u));
+    CHECK(print_one("0xFF", "0x%X", 255u));
+    CHECK(print_one("027", "0%o", 23u));
+    CHECK(print_one("-1", "%hd", (short) -1));
+    // This could be expanded if need be, it's just convenient to do
+    // it this way.
+    if (sizeof(short) == 2) {
+        CHECK(print_one("8000", "%hx", (unsigned short) 0x8000));
+    }
+    CHECK(print_one("0xf0f0", "0x%lx", 0xf0f0ul));
+    CHECK(print_one("0xF0F0", "0x%llX", 0xf0f0ull));
+    CHECK(print_one("27270", "%zu", (size_t) 27270));
+    CHECK(print_one("27270", "%" PRIuSIZE, (size_t) 27270));
+    CHECK(print_one("hello", "he%so", "ll"));
+    CHECK(print_one("(null)", "%s", zero()));
+    CHECK(print_one("0", "%p", (char *) 0));
+    CHECK(print_one("h", "%c", 'h'));
+    CHECK(print_one("1.500000", "%f", 1.5f));
+    CHECK(print_one("1.5", "%g", 1.5));
+
+    CHECK(print_one("2727", "%" PRIu32, (uint32_t) 2727));
+    CHECK(print_one("aa7", "%" PRIx32, (uint32_t) 2727));
+    CHECK(print_one("2727", "%" PRIu64, (uint64_t) 2727));
+    CHECK(print_one("aa7", "%" PRIx64, (uint64_t) 2727));
+
+    return true;
+}
+END_TEST(testPrintf)