diff --git a/ReactCommon/cxxreact/Executor.h b/ReactCommon/cxxreact/Executor.h
index 3f2b103a4..6f2f83fdc 100644
--- a/ReactCommon/cxxreact/Executor.h
+++ b/ReactCommon/cxxreact/Executor.h
@@ -148,6 +148,61 @@ private:
   size_t m_size;
 };
 
+// JSBigString interface implemented by a file-backed mmap region.
+class JSBigFileString : public JSBigString {
+public:
+
+  JSBigFileString(int fd, size_t size, off_t offset = 0)
+    : m_fd   {fd}
+    , m_data {nullptr}
+  {
+    // Offsets given to mmap must be page aligend. We abstract away that
+    // restriction by sending a page aligned offset to mmap, and keeping track
+    // of the offset within the page that we must alter the mmap pointer by to
+    // get the final desired offset.
+    auto ps = getpagesize();
+    auto d  = lldiv(offset, ps);
+
+    m_mapOff  = d.quot;
+    m_pageOff = d.rem;
+    m_size    = size + m_pageOff;
+  }
+
+  ~JSBigFileString() {
+    if (m_data) {
+      munmap((void *)m_data, m_size);
+    }
+    close(m_fd);
+  }
+
+  bool isAscii() const override {
+    return true;
+  }
+
+  const char *c_str() const override {
+    if (!m_data) {
+      m_data = (const char *)mmap(0, m_size, PROT_READ, MAP_SHARED, m_fd, m_mapOff);
+      CHECK(m_data != MAP_FAILED);
+    }
+    return m_data + m_pageOff;
+  }
+
+  size_t size() const override {
+    return m_size - m_pageOff;
+  }
+
+  int fd() const {
+    return m_fd;
+  }
+
+private:
+  int m_fd;                     // The file descriptor being mmaped
+  size_t m_size;                // The size of the mmaped region
+  size_t m_pageOff;             // The offset in the mmaped region to the data.
+  off_t m_mapOff;               // The offset in the file to the mmaped region.
+  mutable const char *m_data;   // Pointer to the mmaped region.
+};
+
 class JSBigOptimizedBundleString : public JSBigString  {
 public:
   enum class Encoding {
@@ -157,7 +212,6 @@ public:
     Utf16,
   };
 
-
   JSBigOptimizedBundleString(int fd, size_t size, const uint8_t sha1[20], Encoding encoding) :
     m_fd(fd),
     m_size(size),
diff --git a/ReactCommon/cxxreact/tests/BUCK b/ReactCommon/cxxreact/tests/BUCK
index fb51122aa..8ea3af385 100644
--- a/ReactCommon/cxxreact/tests/BUCK
+++ b/ReactCommon/cxxreact/tests/BUCK
@@ -1,6 +1,7 @@
 TEST_SRCS = [
   'CxxMessageQueueTest.cpp',
   'jsarg_helpers.cpp',
+  'jsbigstring.cpp',
   'jscexecutor.cpp',
   'jsclogging.cpp',
   'methodcall.cpp',
@@ -35,6 +36,7 @@ if THIS_IS_FBOBJC:
       '-fexceptions',
     ],
     deps = [
+      '//xplat/folly:molly',
       '//xplat/third-party/gmock:gtest',
       react_native_xplat_target('cxxreact:bridge'),
       react_native_xplat_target('jschelpers:jschelpers'),
diff --git a/ReactCommon/cxxreact/tests/jsbigstring.cpp b/ReactCommon/cxxreact/tests/jsbigstring.cpp
new file mode 100644
index 000000000..31e9adc5f
--- /dev/null
+++ b/ReactCommon/cxxreact/tests/jsbigstring.cpp
@@ -0,0 +1,61 @@
+
+// Copyright 2004-present Facebook. All Rights Reserved.
+#include <sys/mman.h>
+#include <fcntl.h>
+
+#include <folly/File.h>
+#include <gtest/gtest.h>
+#include <cxxreact/Executor.h>
+#include <cxxreact/MessageQueueThread.h>
+#include <cxxreact/MethodCall.h>
+
+using namespace facebook;
+using namespace facebook::react;
+
+namespace {
+int tempFileFromString(std::string contents)
+{
+  std::string tmp {getenv("TMPDIR")};
+  tmp += "/temp.XXXXX";
+
+  std::vector<char> tmpBuf {tmp.begin(), tmp.end()};
+  tmpBuf.push_back('\0');
+
+  const int fd = mkstemp(tmpBuf.data());
+  write(fd, contents.c_str(), contents.size() + 1);
+
+  return fd;
+}
+};
+
+TEST(JSBigFileString, MapWholeFileTest) {
+  std::string data {"Hello, world"};
+  const auto size = data.length() + 1;
+
+  // Initialise Big String
+  int fd = tempFileFromString("Hello, world");
+  JSBigFileString bigStr {fd, size};
+
+  // Test
+  ASSERT_EQ(fd, bigStr.fd());
+  ASSERT_STREQ(data.c_str(), bigStr.c_str());
+}
+
+TEST(JSBigFileString, MapPartTest) {
+  std::string data {"Hello, world"};
+
+  // Sub-string to actually map
+  std::string needle {"or"};
+  off_t offset = data.find(needle);
+
+  // Initialise Big String
+  int fd = tempFileFromString(data);
+  JSBigFileString bigStr {fd, needle.size(), offset};
+
+  // Test
+  ASSERT_EQ(fd, bigStr.fd());
+  ASSERT_EQ(needle.length(), bigStr.size());
+  for (unsigned int i = 0; i < needle.length(); ++i) {
+    ASSERT_EQ(needle[i], bigStr.c_str()[i]);
+  }
+}