diff --git a/Makefile b/Makefile
index aa5168c81..9c79b120f 100644
--- a/Makefile
+++ b/Makefile
@@ -56,6 +56,7 @@ all: check \
 
 build/pyodide.asm.js: src/core/main.o src/core/jsimport.o \
 	        src/core/jsproxy.o src/core/js2python.o \
+		src/core/error_handling.o \
 		src/core/pyproxy.o \
 		src/core/python2js.o \
 		src/core/python2js_buffer.o \
diff --git a/src/core/error_handling.c b/src/core/error_handling.c
new file mode 100644
index 000000000..4fc307298
--- /dev/null
+++ b/src/core/error_handling.c
@@ -0,0 +1,25 @@
+#include "error_handling.h"
+#include "Python.h"
+#include "hiwire.h"
+#include "jsproxy.h"
+#include <emscripten.h>
+
+void
+PyodideErr_SetJsError(JsRef err)
+{
+  PyObject* py_err = JsProxy_new_error(err);
+  PyErr_SetObject((PyObject*)(py_err->ob_type), py_err);
+}
+
+int
+error_handling_init()
+{
+  EM_ASM({
+    Module.handle_js_error = function(e){
+      let err = Module.hiwire.new_value(e);
+      PyodideErr_SetJsError(err);
+      Module.hiwire.decref(err);
+}
+});
+return 0;
+}
diff --git a/src/core/error_handling.h b/src/core/error_handling.h
new file mode 100644
index 000000000..14e415183
--- /dev/null
+++ b/src/core/error_handling.h
@@ -0,0 +1,55 @@
+#ifndef ERROR_HANDLING_H
+#define ERROR_HANDLING_H
+
+/**  Wrap EM_JS so that it produces functions that follow the Python return
+ *  conventions. We catch javascript errors and proxy them and use
+ *  `PyErr_SetObject` to hand them off to python. We need two variants, one
+ *  for functions that return pointers / references (return 0)
+ *  the other for functions that return numbers (return -1).
+ */
+
+typedef int errcode;
+
+// Hiwire wants to import us for errcode, so import hiwire after typedef.
+#include "hiwire.h"
+#include <emscripten.h>
+
+int
+error_handling_init();
+
+// WARNING: These wrappers around EM_JS cause macros in body to be expanded.
+// This causes trouble with true and false.
+// In types.h we provide nonstandard definitions:
+// false ==> (!!0)
+// true ==> (!!1)
+// These work as expected in both C and javascript.
+
+// clang-format off
+#define EM_JS_REF(ret, func_name, args, body...)                               \
+  EM_JS(ret, func_name, args, {                                                \
+    /* "use strict";  TODO: enable this. */                                    \
+    try    /* intentionally no braces, body already has them */                \
+      body /* <== body of func */                                              \
+    catch (e) {                                                                \
+        /* Dummied out until calling code is ready to catch these errors */    \
+        throw e;                                                               \
+        Module.handle_js_error(e);                                             \
+        return 0;                                                              \
+    }                                                                          \
+  })
+
+#define EM_JS_NUM(ret, func_name, args, body...)                               \
+  EM_JS(ret, func_name, args, {                                                \
+    /* "use strict";  TODO: enable this. */                                    \
+    try    /* intentionally no braces, body already has them */                \
+      body /* <== body of func */                                              \
+    catch (e) {                                                                \
+        /* Dummied out until calling code is ready to catch these errors */    \
+        throw e;                                                               \
+        Module.handle_js_error(e);                                             \
+        return -1;                                                             \
+    }                                                                          \
+  })
+// clang-format on
+
+#endif // ERROR_HANDLING_H
diff --git a/src/core/hiwire.c b/src/core/hiwire.c
index 8aebb1f9f..6451da2c7 100644
--- a/src/core/hiwire.c
+++ b/src/core/hiwire.c
@@ -1,3 +1,4 @@
+#include "error_handling.h"
 #include <emscripten.h>
 
 #include "hiwire.h"
@@ -102,7 +103,7 @@ EM_JS(int, hiwire_init, (), {
   return 0;
 });
 
-EM_JS(JsRef, hiwire_incref, (JsRef idval), {
+EM_JS_REF(JsRef, hiwire_incref, (JsRef idval), {
   // clang-format off
   if ((idval & 1) === 0) {
     // least significant bit unset ==> idval is a singleton.
@@ -113,15 +114,19 @@ EM_JS(JsRef, hiwire_incref, (JsRef idval), {
   return Module.hiwire.new_value(Module.hiwire.get_value(idval));
 });
 
-EM_JS(void, hiwire_decref, (JsRef idval), { Module.hiwire.decref(idval); });
+EM_JS_NUM(errcode, hiwire_decref, (JsRef idval), {
+  Module.hiwire.decref(idval);
+});
 
-EM_JS(JsRef, hiwire_int, (int val), { return Module.hiwire.new_value(val); });
-
-EM_JS(JsRef, hiwire_double, (double val), {
+EM_JS_REF(JsRef, hiwire_int, (int val), {
   return Module.hiwire.new_value(val);
 });
 
-EM_JS(JsRef, hiwire_string_ucs4, (const char* ptr, int len), {
+EM_JS_REF(JsRef, hiwire_double, (double val), {
+  return Module.hiwire.new_value(val);
+});
+
+EM_JS_REF(JsRef, hiwire_string_ucs4, (const char* ptr, int len), {
   var jsstr = "";
   var idx = ptr / 4;
   for (var i = 0; i < len; ++i) {
@@ -130,7 +135,7 @@ EM_JS(JsRef, hiwire_string_ucs4, (const char* ptr, int len), {
   return Module.hiwire.new_value(jsstr);
 });
 
-EM_JS(JsRef, hiwire_string_ucs2, (const char* ptr, int len), {
+EM_JS_REF(JsRef, hiwire_string_ucs2, (const char* ptr, int len), {
   var jsstr = "";
   var idx = ptr / 2;
   for (var i = 0; i < len; ++i) {
@@ -139,7 +144,7 @@ EM_JS(JsRef, hiwire_string_ucs2, (const char* ptr, int len), {
   return Module.hiwire.new_value(jsstr);
 });
 
-EM_JS(JsRef, hiwire_string_ucs1, (const char* ptr, int len), {
+EM_JS_REF(JsRef, hiwire_string_ucs1, (const char* ptr, int len), {
   var jsstr = "";
   var idx = ptr;
   for (var i = 0; i < len; ++i) {
@@ -148,81 +153,84 @@ EM_JS(JsRef, hiwire_string_ucs1, (const char* ptr, int len), {
   return Module.hiwire.new_value(jsstr);
 });
 
-EM_JS(JsRef, hiwire_string_utf8, (const char* ptr), {
+EM_JS_REF(JsRef, hiwire_string_utf8, (const char* ptr), {
   return Module.hiwire.new_value(UTF8ToString(ptr));
 });
 
-EM_JS(JsRef, hiwire_string_ascii, (const char* ptr), {
+EM_JS_REF(JsRef, hiwire_string_ascii, (const char* ptr), {
   return Module.hiwire.new_value(AsciiToString(ptr));
 });
 
-EM_JS(JsRef, hiwire_bytes, (char* ptr, int len), {
+EM_JS_REF(JsRef, hiwire_bytes, (char* ptr, int len), {
   var bytes = new Uint8ClampedArray(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(bytes);
 });
 
-EM_JS(JsRef, hiwire_int8array, (i8 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_int8array, (i8 * ptr, int len), {
   var array = new Int8Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_uint8array, (u8 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_uint8array, (u8 * ptr, int len), {
   var array = new Uint8Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_int16array, (i16 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_int16array, (i16 * ptr, int len), {
   var array = new Int16Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_uint16array, (u16 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_uint16array, (u16 * ptr, int len), {
   var array = new Uint16Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_int32array, (i32 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_int32array, (i32 * ptr, int len), {
   var array = new Int32Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_uint32array, (u32 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_uint32array, (u32 * ptr, int len), {
   var array = new Uint32Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_float32array, (f32 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_float32array, (f32 * ptr, int len), {
   var array = new Float32Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(JsRef, hiwire_float64array, (f64 * ptr, int len), {
+EM_JS_REF(JsRef, hiwire_float64array, (f64 * ptr, int len), {
   var array = new Float64Array(Module.HEAPU8.buffer, ptr, len);
   return Module.hiwire.new_value(array);
 })
 
-EM_JS(void, hiwire_throw_error, (JsRef idmsg), {
+EM_JS_NUM(errcode, hiwire_throw_error, (JsRef idmsg), {
   var jsmsg = Module.hiwire.get_value(idmsg);
   Module.hiwire.decref(idmsg);
   throw new Error(jsmsg);
 });
 
-EM_JS(JsRef, hiwire_array, (), { return Module.hiwire.new_value([]); });
+EM_JS_REF(JsRef, hiwire_array, (), { return Module.hiwire.new_value([]); });
 
-EM_JS(void, hiwire_push_array, (JsRef idarr, JsRef idval), {
+EM_JS_NUM(errcode, hiwire_push_array, (JsRef idarr, JsRef idval), {
   Module.hiwire.get_value(idarr).push(Module.hiwire.get_value(idval));
 });
 
-EM_JS(JsRef, hiwire_object, (), { return Module.hiwire.new_value({}); });
+EM_JS_REF(JsRef, hiwire_object, (), { return Module.hiwire.new_value({}); });
 
-EM_JS(void, hiwire_push_object_pair, (JsRef idobj, JsRef idkey, JsRef idval), {
-  var jsobj = Module.hiwire.get_value(idobj);
-  var jskey = Module.hiwire.get_value(idkey);
-  var jsval = Module.hiwire.get_value(idval);
-  jsobj[jskey] = jsval;
-});
+EM_JS_NUM(errcode,
+          hiwire_push_object_pair,
+          (JsRef idobj, JsRef idkey, JsRef idval),
+          {
+            var jsobj = Module.hiwire.get_value(idobj);
+            var jskey = Module.hiwire.get_value(idkey);
+            var jsval = Module.hiwire.get_value(idval);
+            jsobj[jskey] = jsval;
+          });
 
-EM_JS(JsRef, hiwire_get_global, (const char* ptrname), {
+EM_JS_REF(JsRef, hiwire_get_global, (const char* ptrname), {
   var jsname = UTF8ToString(ptrname);
   if (jsname in self) {
     return Module.hiwire.new_value(self[jsname]);
@@ -231,7 +239,7 @@ EM_JS(JsRef, hiwire_get_global, (const char* ptrname), {
   }
 });
 
-EM_JS(JsRef, hiwire_get_member_string, (JsRef idobj, const char* ptrkey), {
+EM_JS_REF(JsRef, hiwire_get_member_string, (JsRef idobj, const char* ptrkey), {
   var jsobj = Module.hiwire.get_value(idobj);
   var jskey = UTF8ToString(ptrkey);
   if (jskey in jsobj) {
@@ -241,32 +249,35 @@ EM_JS(JsRef, hiwire_get_member_string, (JsRef idobj, const char* ptrkey), {
   }
 });
 
-EM_JS(void,
-      hiwire_set_member_string,
-      (JsRef idobj, const char* ptrkey, JsRef idval),
-      {
-        var jsobj = Module.hiwire.get_value(idobj);
-        var jskey = UTF8ToString(ptrkey);
-        var jsval = Module.hiwire.get_value(idval);
-        jsobj[jskey] = jsval;
-      });
+EM_JS_NUM(errcode,
+          hiwire_set_member_string,
+          (JsRef idobj, const char* ptrkey, JsRef idval),
+          {
+            var jsobj = Module.hiwire.get_value(idobj);
+            var jskey = UTF8ToString(ptrkey);
+            var jsval = Module.hiwire.get_value(idval);
+            jsobj[jskey] = jsval;
+          });
 
-EM_JS(void, hiwire_delete_member_string, (JsRef idobj, const char* ptrkey), {
-  var jsobj = Module.hiwire.get_value(idobj);
-  var jskey = UTF8ToString(ptrkey);
-  delete jsobj[jskey];
-});
+EM_JS_NUM(errcode,
+          hiwire_delete_member_string,
+          (JsRef idobj, const char* ptrkey),
+          {
+            var jsobj = Module.hiwire.get_value(idobj);
+            var jskey = UTF8ToString(ptrkey);
+            delete jsobj[jskey];
+          });
 
-EM_JS(JsRef, hiwire_get_member_int, (JsRef idobj, int idx), {
+EM_JS_REF(JsRef, hiwire_get_member_int, (JsRef idobj, int idx), {
   var jsobj = Module.hiwire.get_value(idobj);
   return Module.hiwire.new_value(jsobj[idx]);
 });
 
-EM_JS(void, hiwire_set_member_int, (JsRef idobj, int idx, JsRef idval), {
+EM_JS_NUM(errcode, hiwire_set_member_int, (JsRef idobj, int idx, JsRef idval), {
   Module.hiwire.get_value(idobj)[idx] = Module.hiwire.get_value(idval);
 });
 
-EM_JS(JsRef, hiwire_get_member_obj, (JsRef idobj, JsRef ididx), {
+EM_JS_REF(JsRef, hiwire_get_member_obj, (JsRef idobj, JsRef ididx), {
   var jsobj = Module.hiwire.get_value(idobj);
   var jsidx = Module.hiwire.get_value(ididx);
   if (jsidx in jsobj) {
@@ -276,20 +287,23 @@ EM_JS(JsRef, hiwire_get_member_obj, (JsRef idobj, JsRef ididx), {
   }
 });
 
-EM_JS(void, hiwire_set_member_obj, (JsRef idobj, JsRef ididx, JsRef idval), {
-  var jsobj = Module.hiwire.get_value(idobj);
-  var jsidx = Module.hiwire.get_value(ididx);
-  var jsval = Module.hiwire.get_value(idval);
-  jsobj[jsidx] = jsval;
-});
+EM_JS_NUM(errcode,
+          hiwire_set_member_obj,
+          (JsRef idobj, JsRef ididx, JsRef idval),
+          {
+            var jsobj = Module.hiwire.get_value(idobj);
+            var jsidx = Module.hiwire.get_value(ididx);
+            var jsval = Module.hiwire.get_value(idval);
+            jsobj[jsidx] = jsval;
+          });
 
-EM_JS(void, hiwire_delete_member_obj, (JsRef idobj, JsRef ididx), {
+EM_JS_NUM(errcode, hiwire_delete_member_obj, (JsRef idobj, JsRef ididx), {
   var jsobj = Module.hiwire.get_value(idobj);
   var jsidx = Module.hiwire.get_value(ididx);
   delete jsobj[jsidx];
 });
 
-EM_JS(JsRef, hiwire_dir, (JsRef idobj), {
+EM_JS_REF(JsRef, hiwire_dir, (JsRef idobj), {
   var jsobj = Module.hiwire.get_value(idobj);
   var result = [];
   do {
@@ -298,23 +312,23 @@ EM_JS(JsRef, hiwire_dir, (JsRef idobj), {
   return Module.hiwire.new_value(result);
 });
 
-EM_JS(JsRef, hiwire_call, (JsRef idfunc, JsRef idargs), {
+EM_JS_REF(JsRef, hiwire_call, (JsRef idfunc, JsRef idargs), {
   var jsfunc = Module.hiwire.get_value(idfunc);
   var jsargs = Module.hiwire.get_value(idargs);
   return Module.hiwire.new_value(jsfunc.apply(jsfunc, jsargs));
 });
 
-EM_JS(JsRef,
-      hiwire_call_member,
-      (JsRef idobj, const char* ptrname, JsRef idargs),
-      {
-        var jsobj = Module.hiwire.get_value(idobj);
-        var jsname = UTF8ToString(ptrname);
-        var jsargs = Module.hiwire.get_value(idargs);
-        return Module.hiwire.new_value(jsobj[jsname].apply(jsobj, jsargs));
-      });
+EM_JS_REF(JsRef,
+          hiwire_call_member,
+          (JsRef idobj, const char* ptrname, JsRef idargs),
+          {
+            var jsobj = Module.hiwire.get_value(idobj);
+            var jsname = UTF8ToString(ptrname);
+            var jsargs = Module.hiwire.get_value(idargs);
+            return Module.hiwire.new_value(jsobj[jsname].apply(jsobj, jsargs));
+          });
 
-EM_JS(JsRef, hiwire_new, (JsRef idobj, JsRef idargs), {
+EM_JS_REF(JsRef, hiwire_new, (JsRef idobj, JsRef idargs), {
   function newCall(Cls)
   {
     return new (Function.prototype.bind.apply(Cls, arguments));
@@ -325,37 +339,37 @@ EM_JS(JsRef, hiwire_new, (JsRef idobj, JsRef idargs), {
   return Module.hiwire.new_value(newCall.apply(newCall, jsargs));
 });
 
-EM_JS(int, hiwire_get_length, (JsRef idobj), {
+EM_JS_NUM(int, hiwire_get_length, (JsRef idobj), {
   return Module.hiwire.get_value(idobj).length;
 });
 
-EM_JS(bool, hiwire_get_bool, (JsRef idobj), {
+EM_JS_NUM(bool, hiwire_get_bool, (JsRef idobj), {
   var val = Module.hiwire.get_value(idobj);
   // clang-format off
   return (val && (val.length === undefined || val.length)) ? 1 : 0;
   // clang-format on
 });
 
-EM_JS(bool, hiwire_is_function, (JsRef idobj), {
+EM_JS_NUM(bool, hiwire_is_function, (JsRef idobj), {
   // clang-format off
   return typeof Module.hiwire.get_value(idobj) === 'function';
   // clang-format on
 });
 
-EM_JS(JsRef, hiwire_to_string, (JsRef idobj), {
+EM_JS_REF(JsRef, hiwire_to_string, (JsRef idobj), {
   return Module.hiwire.new_value(Module.hiwire.get_value(idobj).toString());
 });
 
-EM_JS(JsRef, hiwire_typeof, (JsRef idobj), {
+EM_JS_REF(JsRef, hiwire_typeof, (JsRef idobj), {
   return Module.hiwire.new_value(typeof Module.hiwire.get_value(idobj));
 });
 
-EM_JS(char*, hiwire_constructor_name, (JsRef idobj), {
+EM_JS_REF(char*, hiwire_constructor_name, (JsRef idobj), {
   return stringToNewUTF8(Module.hiwire.get_value(idobj).constructor.name);
 });
 
 #define MAKE_OPERATOR(name, op)                                                \
-  EM_JS(bool, hiwire_##name, (JsRef ida, JsRef idb), {                         \
+  EM_JS_NUM(bool, hiwire_##name, (JsRef ida, JsRef idb), {                     \
     return (Module.hiwire.get_value(ida) op Module.hiwire.get_value(idb)) ? 1  \
                                                                           : 0; \
   })
@@ -367,7 +381,7 @@ MAKE_OPERATOR(not_equal, !=);
 MAKE_OPERATOR(greater_than, >);
 MAKE_OPERATOR(greater_than_equal, >=);
 
-EM_JS(JsRef, hiwire_next, (JsRef idobj), {
+EM_JS_REF(JsRef, hiwire_next, (JsRef idobj), {
   // clang-format off
   if (idobj === Module.hiwire.UNDEFINED) {
     return Module.hiwire.ERROR;
@@ -378,7 +392,7 @@ EM_JS(JsRef, hiwire_next, (JsRef idobj), {
   // clang-format on
 });
 
-EM_JS(JsRef, hiwire_get_iterator, (JsRef idobj), {
+EM_JS_REF(JsRef, hiwire_get_iterator, (JsRef idobj), {
   // clang-format off
   if (idobj === Module.hiwire.UNDEFINED) {
     return Module.hiwire.ERROR;
@@ -396,37 +410,37 @@ EM_JS(JsRef, hiwire_get_iterator, (JsRef idobj), {
   // clang-format on
 })
 
-EM_JS(bool, hiwire_nonzero, (JsRef idobj), {
+EM_JS_NUM(bool, hiwire_nonzero, (JsRef idobj), {
   var jsobj = Module.hiwire.get_value(idobj);
   // TODO: should this be !== 0?
   return (jsobj != 0) ? 1 : 0;
 });
 
-EM_JS(bool, hiwire_is_typedarray, (JsRef idobj), {
+EM_JS_NUM(bool, hiwire_is_typedarray, (JsRef idobj), {
   var jsobj = Module.hiwire.get_value(idobj);
   // clang-format off
   return (jsobj['byteLength'] !== undefined) ? 1 : 0;
   // clang-format on
 });
 
-EM_JS(bool, hiwire_is_on_wasm_heap, (JsRef idobj), {
+EM_JS_NUM(bool, hiwire_is_on_wasm_heap, (JsRef idobj), {
   var jsobj = Module.hiwire.get_value(idobj);
   // clang-format off
   return (jsobj.buffer === Module.HEAPU8.buffer) ? 1 : 0;
   // clang-format on
 });
 
-EM_JS(int, hiwire_get_byteOffset, (JsRef idobj), {
+EM_JS_NUM(int, hiwire_get_byteOffset, (JsRef idobj), {
   var jsobj = Module.hiwire.get_value(idobj);
   return jsobj['byteOffset'];
 });
 
-EM_JS(int, hiwire_get_byteLength, (JsRef idobj), {
+EM_JS_NUM(int, hiwire_get_byteLength, (JsRef idobj), {
   var jsobj = Module.hiwire.get_value(idobj);
   return jsobj['byteLength'];
 });
 
-EM_JS(void, hiwire_copy_to_ptr, (JsRef idobj, void* ptr), {
+EM_JS_NUM(errcode, hiwire_copy_to_ptr, (JsRef idobj, void* ptr), {
   var jsobj = Module.hiwire.get_value(idobj);
   // clang-format off
   var buffer = (jsobj['buffer'] !== undefined) ? jsobj.buffer : jsobj;
@@ -434,34 +448,34 @@ EM_JS(void, hiwire_copy_to_ptr, (JsRef idobj, void* ptr), {
   Module.HEAPU8.set(new Uint8Array(buffer), ptr);
 });
 
-EM_JS(void,
-      hiwire_get_dtype,
-      (JsRef idobj, char** format_ptr, Py_ssize_t* size_ptr),
-      {
-        if (!Module.hiwire.dtype_map) {
-          let alloc = stringToNewUTF8;
-          Module.hiwire.dtype_map = new Map([
-            [ 'Int8Array', [ alloc('b'), 1 ] ],
-            [ 'Uint8Array', [ alloc('B'), 1 ] ],
-            [ 'Uint8ClampedArray', [ alloc('B'), 1 ] ],
-            [ 'Int16Array', [ alloc('h'), 2 ] ],
-            [ 'Uint16Array', [ alloc('H'), 2 ] ],
-            [ 'Int32Array', [ alloc('i'), 4 ] ],
-            [ 'Uint32Array', [ alloc('I'), 4 ] ],
-            [ 'Float32Array', [ alloc('f'), 4 ] ],
-            [ 'Float64Array', [ alloc('d'), 8 ] ],
-            [ 'ArrayBuffer', [ alloc('B'), 1 ] ], // Default to Uint8;
-          ]);
-        }
-        let jsobj = Module.hiwire.get_value(idobj);
-        let[format_utf8, size] =
-          Module.hiwire.dtype_map.get(jsobj.constructor.name) || [ 0, 0 ];
-        // Store results into arguments
-        setValue(format_ptr, format_utf8, "i8*");
-        setValue(size_ptr, size, "i32");
-      });
+EM_JS_NUM(errcode,
+          hiwire_get_dtype,
+          (JsRef idobj, char** format_ptr, Py_ssize_t* size_ptr),
+          {
+            if (!Module.hiwire.dtype_map) {
+              let alloc = stringToNewUTF8;
+              Module.hiwire.dtype_map = new Map([
+                [ 'Int8Array', [ alloc('b'), 1 ] ],
+                [ 'Uint8Array', [ alloc('B'), 1 ] ],
+                [ 'Uint8ClampedArray', [ alloc('B'), 1 ] ],
+                [ 'Int16Array', [ alloc('h'), 2 ] ],
+                [ 'Uint16Array', [ alloc('H'), 2 ] ],
+                [ 'Int32Array', [ alloc('i'), 4 ] ],
+                [ 'Uint32Array', [ alloc('I'), 4 ] ],
+                [ 'Float32Array', [ alloc('f'), 4 ] ],
+                [ 'Float64Array', [ alloc('d'), 8 ] ],
+                [ 'ArrayBuffer', [ alloc('B'), 1 ] ], // Default to Uint8;
+              ]);
+            }
+            let jsobj = Module.hiwire.get_value(idobj);
+            let[format_utf8, size] =
+              Module.hiwire.dtype_map.get(jsobj.constructor.name) || [ 0, 0 ];
+            // Store results into arguments
+            setValue(format_ptr, format_utf8, "i8*");
+            setValue(size_ptr, size, "i32");
+          });
 
-EM_JS(JsRef, hiwire_subarray, (JsRef idarr, int start, int end), {
+EM_JS_REF(JsRef, hiwire_subarray, (JsRef idarr, int start, int end), {
   var jsarr = Module.hiwire.get_value(idarr);
   var jssub = jsarr.subarray(start, end);
   return Module.hiwire.new_value(jssub);
diff --git a/src/core/hiwire.h b/src/core/hiwire.h
index 7b988e122..7b2ee0f2d 100644
--- a/src/core/hiwire.h
+++ b/src/core/hiwire.h
@@ -32,6 +32,9 @@ struct _JsRefStruct
 
 typedef struct _JsRefStruct* JsRef;
 
+// Error handling will want to see JsRef.
+#include "error_handling.h"
+
 // Special JsRefs for singleton constants.
 // (These must be even because the least significance bit is set to 0 for
 // singleton constants.)
@@ -57,7 +60,7 @@ hiwire_incref(JsRef idval);
 /**
  * Decrease the reference count on an object.
  */
-void
+errcode
 hiwire_decref(JsRef idval);
 
 /**
@@ -276,7 +279,7 @@ hiwire_array();
  * If the user no longer needs the value outside of the array, it is the user's
  * responsibility to decref it.
  */
-void
+errcode
 hiwire_push_array(JsRef idobj, JsRef idval);
 
 /**
@@ -293,7 +296,7 @@ hiwire_object();
  * If the user no longer needs the key or value outside of the object, it is the
  * user's responsibility to decref them.
  */
-void
+errcode
 hiwire_push_object_pair(JsRef idobj, JsRef idkey, JsRef idval);
 
 /**
@@ -302,7 +305,7 @@ hiwire_push_object_pair(JsRef idobj, JsRef idkey, JsRef idval);
  * The message is conventionally a Javascript string, but that is not required.
  * TODO: should be hiwire_set_error.
  */
-void
+errcode
 hiwire_throw_error(JsRef idmsg);
 
 /**
@@ -325,14 +328,14 @@ hiwire_get_member_string(JsRef idobj, const char* ptrname);
 /**
  * Set an object member by string.
  */
-void
+errcode
 hiwire_set_member_string(JsRef idobj, const char* ptrname, JsRef idval);
 
 /**
  * Delete an object member by string.
  *
  */
-void
+errcode
 hiwire_delete_member_string(JsRef idobj, const char* ptrname);
 
 /**
@@ -351,7 +354,7 @@ hiwire_get_member_int(JsRef idobj, int idx);
  * The integer is a C integer, not an id reference to a Javascript integer.
  *
  */
-void
+errcode
 hiwire_set_member_int(JsRef idobj, int idx, JsRef idval);
 
 /**
@@ -366,14 +369,14 @@ hiwire_get_member_obj(JsRef idobj, JsRef ididx);
  * Set an object member by object.
  *
  */
-void
+errcode
 hiwire_set_member_obj(JsRef idobj, JsRef ididx, JsRef idval);
 
 /**
  * Delete an object member by object.
  *
  */
-void
+errcode
 hiwire_delete_member_obj(JsRef idobj, JsRef ididx);
 
 /**
@@ -554,13 +557,13 @@ hiwire_get_byteOffset(JsRef idobj);
  * Copies the buffer contents of a given typed array or buffer into the memory
  * at ptr.
  */
-void
+errcode
 hiwire_copy_to_ptr(JsRef idobj, void* ptr);
 
 /**
  * Get a data type identifier for a given typedarray.
  */
-void
+errcode
 hiwire_get_dtype(JsRef idobj, char** format_ptr, Py_ssize_t* size_ptr);
 
 /**
diff --git a/src/core/js2python.c b/src/core/js2python.c
index 3dec1afb7..9d0c3aef7 100644
--- a/src/core/js2python.c
+++ b/src/core/js2python.c
@@ -1,4 +1,5 @@
 #include "js2python.h"
+#include "error_handling.h"
 
 #include <emscripten.h>
 
@@ -79,8 +80,7 @@ _js2python_error(JsRef id)
 }
 
 // TODO: Add some meaningful order
-
-EM_JS(PyObject*, __js2python, (JsRef id), {
+EM_JS_REF(PyObject*, __js2python, (JsRef id), {
   function __js2python_string(value)
   {
     // The general idea here is to allocate a Python string and then
diff --git a/src/core/main.c b/src/core/main.c
index e6e48030f..8dd51e675 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -3,6 +3,7 @@
 #include <emscripten.h>
 #include <stdalign.h>
 
+#include "error_handling.h"
 #include "hiwire.h"
 #include "js2python.h"
 #include "jsimport.h"
@@ -59,6 +60,7 @@ main(int argc, char** argv)
   }
   Py_DECREF(sys);
 
+  TRY_INIT(error_handling);
   TRY_INIT(js2python);
   TRY_INIT(JsImport);
   TRY_INIT(JsProxy);
diff --git a/src/core/pyproxy.c b/src/core/pyproxy.c
index a4dda3c91..f77ae9675 100644
--- a/src/core/pyproxy.c
+++ b/src/core/pyproxy.c
@@ -1,3 +1,4 @@
+#include "error_handling.h"
 #include <Python.h>
 #include <emscripten.h>
 
@@ -144,7 +145,7 @@ _pyproxy_destroy(PyObject* ptrobj)
   EM_ASM(delete Module.PyProxies[ptrobj];);
 }
 
-EM_JS(JsRef, pyproxy_use, (PyObject * ptrobj), {
+EM_JS_REF(JsRef, pyproxy_use, (PyObject * ptrobj), {
   // Checks if there is already an existing proxy on ptrobj
 
   if (Module.PyProxies.hasOwnProperty(ptrobj)) {
@@ -154,7 +155,7 @@ EM_JS(JsRef, pyproxy_use, (PyObject * ptrobj), {
   return Module.hiwire.ERROR;
 })
 
-EM_JS(JsRef, pyproxy_new, (PyObject * ptrobj), {
+EM_JS_REF(JsRef, pyproxy_new, (PyObject * ptrobj), {
   // Technically, this leaks memory, since we're holding on to a reference
   // to the proxy forever.  But we have that problem anyway since we don't
   // have a destructor in Javascript to free the Python object.
@@ -169,7 +170,7 @@ EM_JS(JsRef, pyproxy_new, (PyObject * ptrobj), {
   return Module.hiwire.new_value(proxy);
 });
 
-EM_JS(int, pyproxy_init, (), {
+EM_JS_NUM(int, pyproxy_init, (), {
   // clang-format off
   Module.PyProxies = {};
   Module.PyProxy = {
diff --git a/src/core/runpython.c b/src/core/runpython.c
index 46bf000d2..c10c57f8b 100644
--- a/src/core/runpython.c
+++ b/src/core/runpython.c
@@ -1,4 +1,5 @@
 #include "runpython.h"
+#include "error_handling.h"
 #include "hiwire.h"
 #include "pyproxy.h"
 #include "python2js.h"
@@ -39,22 +40,25 @@ _runPythonDebug(char* code)
   return id;
 }
 
-EM_JS(int, runpython_init_js, (JsRef pyodide_py_proxy, JsRef globals_proxy), {
-  Module.pyodide_py = Module.hiwire.get_value(pyodide_py_proxy);
-  Module.globals = Module.hiwire.get_value(globals_proxy);
+EM_JS_NUM(int,
+          runpython_init_js,
+          (JsRef pyodide_py_proxy, JsRef globals_proxy),
+          {
+            Module.pyodide_py = Module.hiwire.get_value(pyodide_py_proxy);
+            Module.globals = Module.hiwire.get_value(globals_proxy);
 
-  // Use this to test python code separate from pyproxy.apply.
-  Module.runPythonDebug = function(code)
-  {
-    let pycode = stringToNewUTF8(code);
-    let idresult = Module.__runPythonDebug(pycode);
-    let jsresult = Module.hiwire.get_value(idresult);
-    Module.hiwire.decref(idresult);
-    _free(pycode);
-    return jsresult;
-  };
-  return 0;
-});
+            // Use this to test python code separate from pyproxy.apply.
+            Module.runPythonDebug = function(code)
+            {
+              let pycode = stringToNewUTF8(code);
+              let idresult = Module.__runPythonDebug(pycode);
+              let jsresult = Module.hiwire.get_value(idresult);
+              Module.hiwire.decref(idresult);
+              _free(pycode);
+              return jsresult;
+            };
+            return 0;
+          });
 
 #define QUIT_IF_NULL(x)                                                        \
   do {                                                                         \
diff --git a/src/core/types.h b/src/core/types.h
index c4edae244..479034065 100644
--- a/src/core/types.h
+++ b/src/core/types.h
@@ -4,6 +4,16 @@
 #include "stdbool.h"
 #include "stdint.h"
 
+#undef false
+#undef true
+// These work for both C and javascript.
+// In C !!0 ==> 0 and in javascript !!0 ==> false
+// In C !!1 ==> 1 and in javascript !!1 ==> true
+// clang-format off
+#define false (!!0)
+#define true (!!1)
+// clang-format on
+
 typedef uint8_t u8;
 typedef uint16_t u16;
 typedef uint32_t u32;
@@ -15,4 +25,5 @@ typedef int64_t i64;
 
 typedef float f32;
 typedef double f64;
+
 #endif /* MY_LINUX_TYPES_H */