# Validate `fetch()` Responses Before Parsing JSON

Priority: **Medium** · Effort: **Low** · Risk: **Low**

## Problem

Several client `fetch` call sites parse the response body as JSON **without checking `res.ok`** (or `res.status`). When the server returns an error envelope (e.g. `{ error: "Authentication required" }` with 401) or a non-JSON body (proxy 502, HTML error page), the parsed object is then **assigned directly to application state** and immediately rendered — silently corrupting the UI.

## Affected Locations

| File:Line | Call | Current behavior on error |
|-----------|------|---------------------------|
| `public/queue.js:382` | `M.library = await res.json();` then `M.renderLibrary()` | The entire library silently becomes the error object; render runs on garbage. |
| `public/auth.js:11` | guest session bootstrap | Error body used as the user object. |
| `public/auth.js:101` | `/api/auth/me` | Error body used as the current user. |
| `public/channelSync.js:11` | `/api/channels` list | Error body assigned to `M.channels`; `channels.length === 0` check then behaves confusingly. |

(Other call sites in `playlists.js`, `upload.js`, etc. already do `if (!res.ok)` correctly — follow those as the model.)

## Implementation Plan

### Step 1 — Add a shared helper in `public/utils.js`

Centralize the check so it can't be forgotten again:

```js
M.apiJson = async function (res) {
  if (!res.ok) {
    let detail = "";
    try { detail = (await res.clone().json()).error ?? ""; } catch { /* non-JSON body */ }
    const err = new Error(`Request failed (${res.status})${detail ? ": " + detail : ""}`);
    err.status = res.status;
    err.detail = detail;
    throw err;
  }
  return res.json();
};
```

(`res.clone()` so the original body remains readable if the caller wants it.)

### Step 2 — Update each affected call site

**`queue.js:~380`** (library load):

```js
const res = await fetch("/api/library", { credentials: "include" });
if (res.status === 401) { M.handleAuthRequired?.(); return; }   // or surface login UI
M.library = await M.apiJson(res);
M.renderLibrary();
```

Wrap the surrounding logic in `try/catch` and show a toast on failure (e.g. `M.showToast("Couldn't load library", "error")`) rather than leaving the UI empty/silent. Keep `M.library` as its previous value on failure rather than overwriting with garbage.

**`auth.js:11`** (guest bootstrap) and **`auth.js:101`** (`/api/auth/me`):

```js
const res = await fetch("/api/auth/me", { credentials: "include" });
if (res.status === 401) { /* not logged in / guest expired — drive login UI */ return; }
const user = await M.apiJson(res);
```

**`channelSync.js:11`** (channel list):

```js
const res = await fetch("/api/channels", { credentials: "include" });
if (!res.ok) { M.showToast("Couldn't load channels", "error"); return; }
const channels = await M.apiJson(res);
M.channels = channels;
```

### Step 3 — Audit remaining call sites

Grep for `await res.json()` and `await response.json()` across `public/`. Convert any that lack a preceding `res.ok` / status check to use `M.apiJson`. The ones already guarded can be left or migrated for consistency.

## Validation

- **Simulate 401**: clear the session cookie in DevTools, reload. Confirm the library view does **not** render an error object as tracks, and a login prompt / toast appears instead.
- **Simulate 500/502**: stop the server mid-session and trigger a library reload (e.g. via the refresh path). Confirm a toast shows and the previous library remains visible.
- **Happy path**: confirm normal load of library, channels, and `/api/auth/me` still works after the change.
- **Guest expiry**: let a guest session lapse (or delete it from the DB) and reload; confirm graceful handling rather than a broken render.

## Risk / Rollback

- Behavior change only on the error path; success path is identical.
- One subtlety: previously-silent failures will now surface as toasts. That is the desired behavior, but verify the messages are user-friendly and not noisy in normal flaky-network conditions.
- Rollback = revert the helper and the four call sites; no data or schema impact.
