diff --git a/js/symbols.js b/js/symbols.js
deleted file mode 100644
index 059b8d8..0000000
--- a/js/symbols.js
+++ /dev/null
@@ -1,11 +0,0 @@
-export var SYMBOLS = {
-    0xD010: 'KBD',
-    0xD011: 'KBDCR',
-    0xD012: 'DSP',
-    0xD013: 'DSPCR',
-
-    0xFF1F: 'GETLINE',
-    0xFFEF: 'ECHO',
-    0xFFDC: 'PRBYTE',
-    0xFFE5: 'PRHEX'
-};
diff --git a/js/symbols.ts b/js/symbols.ts
new file mode 100644
index 0000000..bcdbe6b
--- /dev/null
+++ b/js/symbols.ts
@@ -0,0 +1,24 @@
+/* Copyright 2010-2023 Will Scullin <scullin@scullinsteel.com>
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation.  No representations are made about the suitability of this
+ * software for any purpose.  It is provided "as is" without express or
+ * implied warranty.
+ */
+
+import type { address } from "./types";
+
+export const SYMBOLS: Record<address, string> = {
+  0xd010: "KBD",
+  0xd011: "KBDCR",
+  0xd012: "DSP",
+  0xd013: "DSPCR",
+
+  0xff1f: "GETLINE",
+  0xffef: "ECHO",
+  0xffdc: "PRBYTE",
+  0xffe5: "PRHEX",
+};