From 43666c47b550904e5573e28fab6ba9d2c51ddcd3 Mon Sep 17 00:00:00 2001
From: Adam Chlipala <adam@chlipala.net>
Date: Sun, 10 Mar 2019 15:37:40 -0400
Subject: JavaScript string subscripting should raise out-of-bounds exceptions

---
 lib/js/urweb.js | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/js/urweb.js b/lib/js/urweb.js
index 1c296fe1..749e5056 100644
--- a/lib/js/urweb.js
+++ b/lib/js/urweb.js
@@ -2493,6 +2493,8 @@ function id(x) { return x; }
 function sub(s, i) {
     var ch = undefined;
     iterateString(s, function(c, idx) { if (idx == i) { ch = c; return false; }});
+    if (ch == undefined)
+        er("String index " + i + " out of bounds")
     return ch;
 }
 function suf(s, i) {
-- 
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