fix: prevent JSON.stringify on non-JSON payloads

create-fetchival.js

@@ -8,21 +8,27 @@ function query(params) {
     .join('&')}`
 }
 
+const prevenJsonStringifyContentTypes = new Set(['application/x-sentry-envelope'])
+
 function createFetchival({ fetch }) {
   async function _fetch(method, url, opts, data) {
     // Unlike fetchival, don't silently ignore and override
     if (opts.body) throw new Error('unexpected pre-set body option')
 
+    const headers = {
+      Accept: 'application/json',
+      'Content-Type': 'application/json',
+      ...opts.headers,
+    }
+    // Allows some consumers to avoid JSON.stringify for their vendor-specific content types.
+    const shouldStringify = !prevenJsonStringifyContentTypes.has(headers['Content-Type'])
+
     // Unlike fetchival, don't pollute the opts object we were given
     const res = await fetchival.fetch(url, {
       ...opts,
       method,
-      headers: {
-        Accept: 'application/json',
-        'Content-Type': 'application/json',
-        ...opts.headers,
-      },
-      ...(data ? { body: JSON.stringify(data) } : {}),
+      headers,
+      ...(data ? { body: shouldStringify ? JSON.stringify(data) : data } : {}),
     })
 
     if (res.status >= 200 && res.status < 300) {

test/create-fetchival.js

@@ -31,4 +31,48 @@ tape('createFetchival', (t) => {
       t.equals(err.message, 'Couldnt establish connection')
     }
   })
+
+  t.test('does not stringify body for specific Conent-Type headers', async (t) => {
+    let capturedOpts
+    const formData = 'name=John&age=30'
+    const fetch = async (url, opts) => {
+      capturedOpts = opts
+      return { status: 200, json: async () => ({ success: true }) }
+    }
+
+    const fetchival = createFetchival({ fetch })
+
+    await fetchival('https://example.com')('api', {
+      headers: {
+        'Content-Type': 'application/x-sentry-envelope',
+      },
+    }).post(formData)
+
+    t.equals(
+      capturedOpts.body,
+      formData,
+      'body should not be stringified for non-JSON content type'
+    )
+    t.equals(capturedOpts.headers['Content-Type'], 'application/x-sentry-envelope')
+  })
+
+  t.test('applies JSON.stringify to body by default', async (t) => {
+    let capturedOpts
+    const formData = 'name=John&age=30'
+    const fetch = async (url, opts) => {
+      capturedOpts = opts
+      return { status: 200, json: async () => ({ success: true }) }
+    }
+
+    const fetchival = createFetchival({ fetch })
+
+    await fetchival('https://example.com')('api').post(formData)
+
+    t.equals(
+      capturedOpts.body,
+      JSON.stringify(formData),
+      'body should not be stringified for non-JSON content type'
+    )
+    t.equals(capturedOpts.headers['Content-Type'], 'application/json')
+  })
 })