Accept pre-parsed JSON body in HttpError.from()

README.md

@@ -48,12 +48,27 @@ try {
 
 Creates an error with message `"${status} ${statusText}"` and sets `cause` to the response.
 
-### `HttpError.from(response)`
+### `HttpError.from(response, [json])`
 
 Async factory that creates an `HttpError` and captures the response body:
 
 - `err.text` — the response body as a string
 - `err.json` — the parsed JSON (if the body is valid JSON)
 - `err.cause` — the original `Response` object
 
-The original response is not consumed (uses `response.clone()`).
+When called with just a `response`, reads the body via `response.clone().text()` so the original response is not consumed.
+
+When the caller has already consumed the body (via `response.json()`), pass the parsed body as the second argument to skip the body read. Useful for application-level error envelopes on otherwise-ok responses (GraphQL `data.errors[]`, REST 200-with-`errors[]`, etc.):
+
+```javascript
+const response = await fetch('https://api.example.com/graphql', { /* ... */ });
+const json = await response.json();
+
+if (json.errors?.length) {
+    const err = await HttpError.from(response, json);
+    err.message = json.errors.map(e => e.message).join('; ');
+    throw err;
+}
+```
+
+In that mode, `.json` is set to the supplied body, `.text` to `JSON.stringify(json)`, and the message defaults to `"${status} ${statusText}"` — override it after construction if you want a different one.

index.js

@@ -13,11 +13,22 @@ class HttpError extends Error {
 
     /**
      * Create an HttpError from a fetch Response, capturing the response body as text and JSON.
+     * Pass `json` when the caller has already consumed the body via `response.json()` — useful
+     * for application-level error envelopes on otherwise-ok responses (GraphQL data.errors[],
+     * REST 200 with errors[], etc.) — to skip the body read.
      * @param {Response} response - The fetch Response object.
+     * @param {*} [json] - The already-parsed JSON body, if the caller has it.
      * @returns {Promise<HttpError>} Error with text and json properties.
      */
-    static async from(response) {
+    static async from(response, json) {
         const err = new HttpError(response);
+
+        if (json !== undefined) {
+            err.json = json;
+            err.text = JSON.stringify(json);
+            return err;
+        }
+
         err.text = await response.clone().text().catch(() => {});
 
         if (err.text) {

test/index.js

@@ -49,4 +49,26 @@ test('HttpError', { concurrency: true }, async (t) => {
 
         assert.strictEqual(text, 'body');
     });
+
+    t.test('should use a pre-parsed JSON body when passed as the second argument', async () => {
+        const response = new Response('', { status: 200, statusText: 'OK' });
+        const json = { errors: [{ message: 'Invalid account number' }] };
+        const err = await HttpError.from(response, json);
+
+        assert.strictEqual(err.name, 'HttpError');
+        assert.strictEqual(err.message, '200 OK');
+        assert.strictEqual(err.cause, response);
+        assert.deepStrictEqual(err.json, json);
+        assert.strictEqual(err.text, '{"errors":[{"message":"Invalid account number"}]}');
+    });
+
+    t.test('should let the caller override the message after construction', async () => {
+        const response = new Response('', { status: 200, statusText: 'OK' });
+        const json = { errors: [{ message: 'Invalid account number' }] };
+        const err = await HttpError.from(response, json);
+        err.message = json.errors.map(e => e.message).join('; ');
+
+        assert.strictEqual(err.message, 'Invalid account number');
+        assert.deepStrictEqual(err.json, json);
+    });
 });