Polyfill Array.prototype.at mainly for Safari < 15.4

src/index.tsx

@@ -3,6 +3,8 @@
  *
  * SPDX-License-Identifier: MIT
  */
+// Before everything else so the shims are in place for module initialisation.
+import "./polyfills";
 // The CSS entry point: declares the cascade-layer order Panda's PostCSS
 // plugin fills, and the vendor-layer imports. First so app styles cascade
 // after it.

src/polyfills.test.ts

@@ -0,0 +1,39 @@
+/**
+ * (c) 2026, Micro:bit Educational Foundation and contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+import { describe, expect, it } from "vitest";
+import { at } from "./polyfills";
+
+describe("at", () => {
+  const call = <T>(target: ArrayLike<T>, index: number) =>
+    at.call(target, index) as T | undefined;
+
+  it("indexes from the start", () => {
+    expect(call(["a", "b", "c"], 0)).toEqual("a");
+    expect(call(["a", "b", "c"], 2)).toEqual("c");
+  });
+
+  it("indexes from the end", () => {
+    expect(call(["a", "b", "c"], -1)).toEqual("c");
+    expect(call(["a", "b", "c"], -3)).toEqual("a");
+  });
+
+  it("returns undefined when out of range", () => {
+    expect(call(["a"], 1)).toBeUndefined();
+    expect(call(["a"], -2)).toBeUndefined();
+    expect(call([], -1)).toBeUndefined();
+  });
+
+  it("truncates and coerces the index", () => {
+    expect(call(["a", "b", "c"], 1.9)).toEqual("b");
+    expect(call(["a", "b", "c"], -1.9)).toEqual("c");
+    expect(call(["a", "b", "c"], NaN)).toEqual("a");
+    expect(call(["a", "b", "c"], undefined as unknown as number)).toEqual("a");
+  });
+
+  it("works on strings", () => {
+    expect(call("abc", -1)).toEqual("c");
+  });
+});

src/polyfills.ts

@@ -0,0 +1,21 @@
+/**
+ * (c) 2026, Micro:bit Educational Foundation and contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+// Remove when we drop Safari < 15.4.
+// Also seen helping some v old Chrome but not reasonable to maintain support here past when we drop older Safari.
+export function at<T>(this: ArrayLike<T>, index: number): T | undefined {
+  const i = Math.trunc(index) || 0;
+  return this[i < 0 ? this.length + i : i];
+}
+
+for (const proto of [Array.prototype, String.prototype]) {
+  if (!(proto as { at?: unknown }).at) {
+    Object.defineProperty(proto, "at", {
+      value: at,
+      writable: true,
+      configurable: true,
+    });
+  }
+}