diff --git a/crates/js/src/builtins.rs b/crates/js/src/builtins.rs
index 7016e690d..40131ef07 100644
--- a/crates/js/src/builtins.rs
+++ b/crates/js/src/builtins.rs
@@ -227,6 +227,20 @@ impl Builder {
         Ok(())
     }
 
+    /// Appends `units` `count` times. Nothing is written for an empty `units`, however large
+    /// `count` is.
+    fn push_repeated(&mut self, interp: &mut Interp, units: &[u16], count: u64) -> JsResult<()> {
+        if units.is_empty() || count == 0 {
+            return Ok(());
+        }
+        let more = usize::try_from(count).unwrap_or(usize::MAX).saturating_mul(units.len());
+        self.make_room(interp, more)?;
+        for _ in 0..count {
+            self.units.extend_from_slice(units);
+        }
+        Ok(())
+    }
+
     fn push_str(&mut self, interp: &mut Interp, s: &str) -> JsResult<()> {
         let units: Vec<u16> = s.encode_utf16().collect();
         self.push_units(interp, &units)
diff --git a/crates/js/src/builtins/array.rs b/crates/js/src/builtins/array.rs
index 1361f5599..ad034c64e 100644
--- a/crates/js/src/builtins/array.rs
+++ b/crates/js/src/builtins/array.rs
@@ -291,15 +291,11 @@ fn join_elements(interp: &mut Interp, o: ObjId, len: u32, sep: &[u16], item: fn(
     while let Some(k) = it.next(interp)? {
         let v = get_at(interp, o, k as f64)?;
         let Some(s) = item(interp, v)? else { continue };
-        for _ in seps..k {
-            out.push_units(interp, sep)?;
-        }
+        out.push_repeated(interp, sep, k.saturating_sub(seps))?;
         seps = k;
         out.push_units(interp, &s.flat())?;
     }
-    for _ in seps..u64::from(len - 1) {
-        out.push_units(interp, sep)?;
-    }
+    out.push_repeated(interp, sep, u64::from(len - 1).saturating_sub(seps))?;
     Ok(out.finish())
 }
 
diff --git a/crates/js/src/builtins_tests.rs b/crates/js/src/builtins_tests.rs
index 749454756..f031c70dc 100644
--- a/crates/js/src/builtins_tests.rs
+++ b/crates/js/src/builtins_tests.rs
@@ -231,6 +231,8 @@ fn sparse_arrays_of_any_length_cost_only_their_elements() {
     let cases = [
         ("var a = new Array(4294967295); a.join('')", JsValue::String(String::new())),
         ("var a = []; a[4294967294] = 'z'; a[3] = 'y'; a.join('').length", JsValue::Number(2.0)),
+        // An empty separator is not written once per hole, so a thousand such joins finish.
+        ("var b = []; b[4294967294] = 'z'; for (var i = 0; i < 1000; i++) new Array(4294967295).join('') + b.join(''); i", JsValue::Number(1000.0)),
         ("var a = []; a[4294967294] = 2; a[7] = 1; a.sort(); a[0] * 10 + a[1] + (a.length === 4294967295 ? 0 : 100)", JsValue::Number(12.0)),
         ("var a = []; a[4294967294] = 1; a.indexOf(1)", JsValue::Number(4_294_967_294.0)),
         ("var a = []; a[4294967294] = 1; a.lastIndexOf(1, -1) + a.lastIndexOf(2)", JsValue::Number(4_294_967_293.0)),