diff --git a/dlls/jscript/array.c b/dlls/jscript/array.c
index eda49cbf6a1..d17b6051f70 100644
--- a/dlls/jscript/array.c
+++ b/dlls/jscript/array.c
@@ -103,7 +103,7 @@ static HRESULT get_length(script_ctx_t *ctx, vdisp_t *vdisp, jsdisp_t **jsthis,
 static HRESULT set_length(jsdisp_t *obj, DWORD length)
 {
     if(is_class(obj, JSCLASS_ARRAY)) {
-        ((ArrayInstance*)obj)->length = length;
+        array_from_jsdisp(obj)->length = length;
         return S_OK;
     }
 
@@ -191,7 +191,7 @@ static HRESULT concat_obj(jsdisp_t *array, IDispatch *obj, DWORD *len)
     jsobj = iface_to_jsdisp(obj);
     if(jsobj) {
         if(is_class(jsobj, JSCLASS_ARRAY)) {
-            hres = concat_array(array, (ArrayInstance*)jsobj, len);
+            hres = concat_array(array, array_from_jsdisp(jsobj), len);
             jsdisp_release(jsobj);
             return hres;
         }
@@ -1022,7 +1022,7 @@ static void Array_destructor(jsdisp_t *dispex)
 
 static void Array_on_put(jsdisp_t *dispex, const WCHAR *name)
 {
-    ArrayInstance *array = (ArrayInstance*)dispex;
+    ArrayInstance *array = array_from_jsdisp(dispex);
     const WCHAR *ptr = name;
     DWORD id = 0;
 
diff --git a/dlls/jscript/date.c b/dlls/jscript/date.c
index a66a2864c8f..21a359a10f1 100644
--- a/dlls/jscript/date.c
+++ b/dlls/jscript/date.c
@@ -2424,7 +2424,7 @@ static HRESULT DateConstr_value(script_ctx_t *ctx, vdisp_t *jsthis, WORD flags,
             if(FAILED(hres))
                 return hres;
 
-            di = (DateInstance*)date;
+            di = date_from_jsdisp(date);
             di->time = utc(di->time, di);
         }
         }
diff --git a/dlls/jscript/function.c b/dlls/jscript/function.c
index 04dcb18d923..8480a3bf1a0 100644
--- a/dlls/jscript/function.c
+++ b/dlls/jscript/function.c
@@ -81,7 +81,7 @@ static HRESULT Arguments_value(script_ctx_t *ctx, vdisp_t *jsthis, WORD flags, u
 
 static void Arguments_destructor(jsdisp_t *jsdisp)
 {
-    ArgumentsInstance *arguments = (ArgumentsInstance*)jsdisp;
+    ArgumentsInstance *arguments = arguments_from_jsdisp(jsdisp);
 
     TRACE("(%p)\n", arguments);
 
@@ -98,7 +98,7 @@ static void Arguments_destructor(jsdisp_t *jsdisp)
 
 static unsigned Arguments_idx_length(jsdisp_t *jsdisp)
 {
-    ArgumentsInstance *arguments = (ArgumentsInstance*)jsdisp;
+    ArgumentsInstance *arguments = arguments_from_jsdisp(jsdisp);
     return arguments->argc;
 }
 
@@ -113,7 +113,7 @@ static jsval_t *get_argument_ref(ArgumentsInstance *arguments, unsigned idx)
 
 static HRESULT Arguments_idx_get(jsdisp_t *jsdisp, unsigned idx, jsval_t *r)
 {
-    ArgumentsInstance *arguments = (ArgumentsInstance*)jsdisp;
+    ArgumentsInstance *arguments = arguments_from_jsdisp(jsdisp);
     jsval_t *ref;
 
     TRACE("%p[%u]\n", arguments, idx);
@@ -127,7 +127,7 @@ static HRESULT Arguments_idx_get(jsdisp_t *jsdisp, unsigned idx, jsval_t *r)
 
 static HRESULT Arguments_idx_put(jsdisp_t *jsdisp, unsigned idx, jsval_t val)
 {
-    ArgumentsInstance *arguments = (ArgumentsInstance*)jsdisp;
+    ArgumentsInstance *arguments = arguments_from_jsdisp(jsdisp);
     jsval_t *ref;
     HRESULT hres;
 
@@ -327,7 +327,7 @@ HRESULT Function_invoke(jsdisp_t *func_this, IDispatch *jsthis, WORD flags, unsi
     TRACE("func %p this %p\n", func_this, jsthis);
 
     assert(is_class(func_this, JSCLASS_FUNCTION));
-    function = (FunctionInstance*)func_this;
+    function = function_from_jsdisp(func_this);
 
     flags &= ~DISPATCH_JSCRIPT_INTERNAL_MASK;
     if(function->value_proc)
@@ -530,7 +530,7 @@ HRESULT Function_value(script_ctx_t *ctx, vdisp_t *jsthis, WORD flags, unsigned
         return E_FAIL;
     }
 
-    function = (FunctionInstance*)jsthis->u.jsdisp;
+    function = function_from_jsdisp(jsthis->u.jsdisp);
 
     assert(function->value_proc != NULL);
     return invoke_value_proc(ctx, function, NULL, flags, argc, argv, r);
@@ -577,7 +577,7 @@ static HRESULT Function_get_arguments(script_ctx_t *ctx, jsdisp_t *jsthis, jsval
 
 static void Function_destructor(jsdisp_t *dispex)
 {
-    FunctionInstance *This = (FunctionInstance*)dispex;
+    FunctionInstance *This = function_from_jsdisp(dispex);
 
     if(This->code)
         release_bytecode(This->code);
diff --git a/dlls/jscript/jsregexp.c b/dlls/jscript/jsregexp.c
index a9924eecf9f..5798635cb87 100644
--- a/dlls/jscript/jsregexp.c
+++ b/dlls/jscript/jsregexp.c
@@ -125,7 +125,7 @@ static HRESULT do_regexp_match_next(script_ctx_t *ctx, RegExpInstance *regexp,
 HRESULT regexp_match_next(script_ctx_t *ctx, jsdisp_t *dispex,
         DWORD rem_flags, jsstr_t *jsstr, match_state_t **ret)
 {
-    RegExpInstance *regexp = (RegExpInstance*)dispex;
+    RegExpInstance *regexp = regexp_from_jsdisp(dispex);
     match_state_t *match;
     heap_pool_t *mark;
     const WCHAR *str;
@@ -182,7 +182,7 @@ HRESULT regexp_match_next(script_ctx_t *ctx, jsdisp_t *dispex,
 static HRESULT regexp_match(script_ctx_t *ctx, jsdisp_t *dispex, jsstr_t *jsstr, BOOL gflag,
         match_result_t **match_result, DWORD *result_cnt)
 {
-    RegExpInstance *This = (RegExpInstance*)dispex;
+    RegExpInstance *This = regexp_from_jsdisp(dispex);
     match_result_t *ret = NULL;
     match_state_t *result;
     DWORD i=0, ret_size = 0;
@@ -596,7 +596,7 @@ static HRESULT RegExp_value(script_ctx_t *ctx, vdisp_t *jsthis, WORD flags, unsi
 
 static void RegExp_destructor(jsdisp_t *dispex)
 {
-    RegExpInstance *This = (RegExpInstance*)dispex;
+    RegExpInstance *This = regexp_from_jsdisp(dispex);
 
     if(This->jsregexp)
         regexp_destroy(This->jsregexp);
@@ -708,7 +708,7 @@ HRESULT create_regexp_var(script_ctx_t *ctx, jsval_t src_arg, jsval_t *flags_arg
         obj = iface_to_jsdisp(get_object(src_arg));
         if(obj) {
             if(is_class(obj, JSCLASS_REGEXP)) {
-                RegExpInstance *regexp = (RegExpInstance*)obj;
+                RegExpInstance *regexp = regexp_from_jsdisp(obj);
 
                 hres = create_regexp(ctx, regexp->str, regexp->jsregexp->flags, ret);
                 jsdisp_release(obj);
@@ -754,7 +754,7 @@ HRESULT regexp_string_match(script_ctx_t *ctx, jsdisp_t *re, jsstr_t *jsstr, jsv
     static const WCHAR inputW[] = {'i','n','p','u','t',0};
     static const WCHAR lastIndexW[] = {'l','a','s','t','I','n','d','e','x',0};
 
-    RegExpInstance *regexp = (RegExpInstance*)re;
+    RegExpInstance *regexp = regexp_from_jsdisp(re);
     match_result_t *match_result;
     unsigned match_cnt, i;
     const WCHAR *str;
diff --git a/dlls/jscript/string.c b/dlls/jscript/string.c
index b5c0197e1df..ad978f245cb 100644
--- a/dlls/jscript/string.c
+++ b/dlls/jscript/string.c
@@ -111,7 +111,7 @@ static HRESULT get_string_flat_val(script_ctx_t *ctx, vdisp_t *jsthis, jsstr_t *
 
 static HRESULT String_get_length(script_ctx_t *ctx, jsdisp_t *jsthis, jsval_t *r)
 {
-    StringInstance *string = (StringInstance*)jsthis;
+    StringInstance *string = string_from_jsdisp(jsthis);
 
     TRACE("%p\n", jsthis);
 
@@ -1480,7 +1480,7 @@ static HRESULT String_localeCompare(script_ctx_t *ctx, vdisp_t *jsthis, WORD fla
 
 static HRESULT String_get_value(script_ctx_t *ctx, jsdisp_t *jsthis, jsval_t *r)
 {
-    StringInstance *This = (StringInstance*)jsthis;
+    StringInstance *This = string_from_jsdisp(jsthis);
 
     TRACE("\n");
 
@@ -1490,7 +1490,7 @@ static HRESULT String_get_value(script_ctx_t *ctx, jsdisp_t *jsthis, jsval_t *r)
 
 static void String_destructor(jsdisp_t *dispex)
 {
-    StringInstance *This = (StringInstance*)dispex;
+    StringInstance *This = string_from_jsdisp(dispex);
 
     jsstr_release(This->str);
     heap_free(This);
@@ -1498,7 +1498,7 @@ static void String_destructor(jsdisp_t *dispex)
 
 static unsigned String_idx_length(jsdisp_t *jsdisp)
 {
-    StringInstance *string = (StringInstance*)jsdisp;
+    StringInstance *string = string_from_jsdisp(jsdisp);
 
     /*
      * NOTE: For invoke version < 2, indexed array is not implemented at all.
@@ -1512,7 +1512,7 @@ static unsigned String_idx_length(jsdisp_t *jsdisp)
 
 static HRESULT String_idx_get(jsdisp_t *jsdisp, unsigned idx, jsval_t *r)
 {
-    StringInstance *string = (StringInstance*)jsdisp;
+    StringInstance *string = string_from_jsdisp(jsdisp);
     jsstr_t *ret;
 
     ret = jsstr_substr(string->str, idx, 1);