From 3cdc7af9ff55461a3cb66be9507ba081808265fa Mon Sep 17 00:00:00 2001
From: Nigel Ihza Firdausy <106657905+Ni-zav@users.noreply.github.com>
Date: Sun, 11 Oct 2026 21:41:21 +0700
Subject: [PATCH] fix(script): implement ExtendScript String.substr
 compatibility (#840)

---
 crates/script/src/prelude.js | 19 +++++++++++++++++++
 crates/script/src/tests.rs   | 22 ++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/crates/script/src/prelude.js b/crates/script/src/prelude.js
index f50a968c..48ec1bb2 100644
--- a/crates/script/src/prelude.js
+++ b/crates/script/src/prelude.js
@@ -27,6 +27,25 @@ function __str(v) {
   return String(v);
 }
 
+// ExtendScript includes the ES3 String.substr(start, length) method, which Boa
+// does not currently provide. Follow its legacy length (not end-index) semantics.
+if (typeof String.prototype.substr !== "function") {
+  Object.defineProperty(String.prototype, "substr", {
+    value: function (start, length) {
+      if (this === null || this === undefined) throw new TypeError("String.substr called on null or undefined");
+      var text = String(this), n = text.length;
+      var from = Number(start);
+      if (isNaN(from)) from = 0;
+      // ToIntegerOrInfinity, then clamp positive or negative offsets.
+      from = from < 0 ? Math.max(n + Math.ceil(from), 0) : Math.min(Math.floor(from), n);
+      var count = length === undefined ? n - from : Number(length);
+      if (isNaN(count) || count <= 0) return "";
+      return text.slice(from, from + count);
+    },
+    writable: true, configurable: true
+  });
+}
+
 // ------------------------------------------------------------------ enums
 
 function __enum(name, keys, base) {
diff --git a/crates/script/src/tests.rs b/crates/script/src/tests.rs
index b50a7bfb..ceb72e56 100644
--- a/crates/script/src/tests.rs
+++ b/crates/script/src/tests.rs
@@ -730,3 +730,25 @@ fn import_options_sequence_picks_one_still_or_the_run() {
     assert_eq!(kind_of(&s, "shot_[0001-0003].png"), FootageKind::Sequence);
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+#[test]
+fn extendscript_substr_keeps_start_and_length_semantics() {
+    let mut s = session();
+    let o = ok(
+        &mut s,
+        r#"[
+          "abcdef".substr(0, 2),
+          "abcdef".substr(2, 3),
+          "abcdef".substr(-2),
+          "abcdef".substr(-20, 2),
+          "abcdef".substr(2, 0),
+          "abcdef".substr(2, -3),
+          "abcdef".substr(2.9, 2.9),
+          "abcdef".substr(3),
+          "abcdef".substr(30, 2),
+          "abcdef".substr(NaN, 2),
+          "abcdef".substr(2, NaN)
+        ]"#,
+    );
+    assert_eq!(o.result, json!(["ab", "cde", "ef", "ab", "", "", "cd", "def", "", "ab", ""]));
+}