From fe165fb43d1e2453bfaa86fec7c403e7c2c12b39 Mon Sep 17 00:00:00 2001
From: Gabriel Costa <gabrielcg@proton.me>
Date: Mon, 2 Nov 2026 15:06:13 +0000
Subject: [PATCH] feat: Download helper lib

Signed-off-by: Gabriel Costa <gabrielcg@proton.me>
---
 src/lib/download.test.ts | 55 ++++++++++++++++++++++++++++++++++++++++
 src/lib/download.ts      | 13 ++++++++++
 2 files changed, 68 insertions(+)
 create mode 100644 src/lib/download.test.ts
 create mode 100644 src/lib/download.ts

diff --git a/src/lib/download.test.ts b/src/lib/download.test.ts
new file mode 100644
index 00000000..0d089367
--- /dev/null
+++ b/src/lib/download.test.ts
@@ -0,0 +1,55 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { downloadJson } from "./download";
+
+describe("downloadJson", () => {
+  let createObjectURL: ReturnType<typeof vi.fn>;
+  let revokeObjectURL: ReturnType<typeof vi.fn>;
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+    // jsdom has no object URL support.
+    createObjectURL = vi.fn().mockReturnValue("blob:test-url");
+    revokeObjectURL = vi.fn();
+    Object.assign(URL, { createObjectURL, revokeObjectURL });
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    vi.restoreAllMocks();
+  });
+
+  it("downloads a pretty-printed JSON blob under the given filename", async () => {
+    const click = vi.spyOn(HTMLAnchorElement.prototype, "click").mockImplementation(function (
+      this: HTMLAnchorElement,
+    ) {
+      expect(this.href).toBe("blob:test-url");
+      expect(this.download).toBe("export.json");
+      expect(this.isConnected).toBe(true);
+    });
+
+    downloadJson({ a: 1 }, "export.json");
+
+    expect(click).toHaveBeenCalledOnce();
+    const blob = createObjectURL.mock.calls[0][0] as Blob;
+    expect(blob.type).toBe("application/json");
+    // jsdom's Blob is only readable through its own FileReader, which needs real timers.
+    vi.useRealTimers();
+    const text = await new Promise<string>((resolve) => {
+      const reader = new FileReader();
+      reader.onload = () => resolve(reader.result as string);
+      reader.readAsText(blob);
+    });
+    expect(text).toBe('{\n  "a": 1\n}');
+  });
+
+  it("removes the link and revokes the URL after the click", () => {
+    vi.spyOn(HTMLAnchorElement.prototype, "click").mockImplementation(() => {});
+
+    downloadJson({}, "x.json");
+
+    expect(document.querySelector("a[download]")).toBeNull();
+    expect(revokeObjectURL).not.toHaveBeenCalled();
+    vi.runAllTimers();
+    expect(revokeObjectURL).toHaveBeenCalledWith("blob:test-url");
+  });
+});
diff --git a/src/lib/download.ts b/src/lib/download.ts
new file mode 100644
index 00000000..01c05457
--- /dev/null
+++ b/src/lib/download.ts
@@ -0,0 +1,13 @@
+/** Saves `data` as a pretty-printed JSON file via a temporary object URL. */
+export function downloadJson(data: unknown, filename: string): void {
+  const blob = new Blob([JSON.stringify(data, null, 2)], { type: "application/json" });
+  const url = URL.createObjectURL(blob);
+  const link = document.createElement("a");
+  link.href = url;
+  link.download = filename;
+  document.body.appendChild(link);
+  link.click();
+  link.remove();
+  // Deferred: some browsers (Firefox, Safari) cancel the download if revoked synchronously.
+  setTimeout(() => URL.revokeObjectURL(url), 0);
+}