diff --git a/Libraries/LibJS/Runtime/Value.cpp b/Libraries/LibJS/Runtime/Value.cpp
index 675a938f14d..66582e4622a 100644
--- a/Libraries/LibJS/Runtime/Value.cpp
+++ b/Libraries/LibJS/Runtime/Value.cpp
@@ -49,9 +49,13 @@ String Value::to_string() const
     if (is_undefined())
         return "undefined";
 
-    if (is_number())
+    if (is_number()) {
+        if (is_nan())
+            return "NaN";
+
         // FIXME: This needs improvement.
         return String::number((i32)as_double());
+    }
 
     if (is_object())
         return as_object()->to_primitive(Object::PreferredType::String).to_string();
@@ -108,14 +112,10 @@ Value Value::to_number() const
         if (ok)
             return Value(parsed_int);
 
-        //FIXME: Implement 'NaN'
-        ASSERT_NOT_REACHED();
-
-        break;
+        return js_nan();
     }
     case Type::Undefined:
-        //FIXME: Implement 'NaN'
-        ASSERT_NOT_REACHED();
+        return js_nan();
     case Type::Object:
         return m_value.as_object->to_primitive(Object::PreferredType::Number).to_number();
     }
diff --git a/Libraries/LibJS/Runtime/Value.h b/Libraries/LibJS/Runtime/Value.h
index 5dfe6001016..8e2a1daeaff 100644
--- a/Libraries/LibJS/Runtime/Value.h
+++ b/Libraries/LibJS/Runtime/Value.h
@@ -53,6 +53,8 @@ public:
     bool is_cell() const { return is_string() || is_object(); }
     bool is_array() const;
 
+    bool is_nan() const { return is_number() && __builtin_isnan( as_double()); }
+
     Value()
         : m_type(Type::Undefined)
     {
@@ -166,6 +168,11 @@ inline Value js_null()
     return Value(Value::Type::Null);
 }
 
+inline Value js_nan()
+{
+    return Value(__builtin_nan(""));
+}
+
 Value greater_than(Value lhs, Value rhs);
 Value greater_than_equals(Value lhs, Value rhs);
 Value less_than(Value lhs, Value rhs);
diff --git a/Libraries/LibJS/Tests/NaN-basic.js b/Libraries/LibJS/Tests/NaN-basic.js
new file mode 100644
index 00000000000..1f849beefe8
--- /dev/null
+++ b/Libraries/LibJS/Tests/NaN-basic.js
@@ -0,0 +1,9 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    var nan = undefined + 1;
+    assert(nan + "" == "NaN");
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}