Adding backwards compatibility for those passing a FormData instance as the previously unexposed second parameter

Author: therealparmesh

index.js

@@ -29,7 +29,17 @@ function isFile (value) {
     typeof value.name === 'string'
 }
 
+function isFormData (value) {
+  return value instanceof FormData
+}
+
 function objectToFormData (obj, cfg, fd, pre) {
+  if (isFormData(cfg)) {
+    pre = fd
+    fd = cfg
+    cfg = null
+  }
+
   cfg = cfg || {}
   cfg.indices = cfg.indices || false
   fd = fd || new FormData()

test/index.js

@@ -338,3 +338,19 @@ test('File', t => {
   ])
   t.is(formData.get('foo'), foo)
 })
+
+test('FormData instance as second parameter', t => {
+  const foo = 'foo'
+  const data = new FormData()
+  const formData = objectToFormData({
+    foo
+  }, data)
+
+  t.true(formData.append.calledOnce)
+  t.deepEqual(formData.append.getCall(0).args, [
+    'foo',
+    foo
+  ])
+  t.is(formData.get('foo'), foo)
+  t.is(data, formData)
+})