Browser compat fix.

package.json

@@ -44,6 +44,7 @@
     "@octetstream/eslint-config": "6.2.2",
     "@types/mime-types": "2.1.1",
     "@types/node": "18.7.23",
+    "@types/sinon": "^10.0.13",
     "@typescript-eslint/eslint-plugin": "5.38.1",
     "@typescript-eslint/parser": "5.38.1",
     "ava": "4.3.3",
@@ -60,8 +61,10 @@
     "husky": "8.0.1",
     "lint-staged": "13.0.3",
     "pinst": "3.0.0",
+    "sinon": "^14.0.2",
     "ts-node": "10.9.1",
     "ttypescript": "1.5.13",
-    "typescript": "4.8.4"
+    "typescript": "4.8.4",
+    "web-streams-polyfill": "4.0.0-beta.3"
   }
 }

src/FormDataEncoder.ts

@@ -1,5 +1,6 @@
 /* eslint-disable no-restricted-globals */
 import type {RawHeaders, FormDataEncoderHeaders} from "./util/Headers.js"
+import {getStreamIterator} from "./util/getStreamIterator.js"
 import {createBoundary} from "./util/createBoundary.js"
 import {normalizeValue} from "./util/normalizeValue.js"
 import {isPlainObject} from "./util/isPlainObject.js"
@@ -336,7 +337,7 @@ export class FormDataEncoder {
   async* encode(): AsyncGenerator<Uint8Array, void, undefined> {
     for (const part of this.values()) {
       if (isFile(part)) {
-        yield* part.stream()
+        yield* getStreamIterator(part.stream())
       } else {
         yield part
       }

src/util/getStreamIterator.test.ts

@@ -0,0 +1,51 @@
+import test from "ava"
+
+import {ReadableStream} from "web-streams-polyfill"
+import {stub} from "sinon"
+
+import {getStreamIterator} from "./getStreamIterator.js"
+
+test(
+  "Returns readable stream as is, if it implements Symbol.asyncIterator",
+
+  t => {
+    const stream = new ReadableStream()
+
+    t.is(getStreamIterator(stream), stream)
+  }
+)
+
+test(
+  "Returns fallback when given stream does not implement Symbol.asyncIterator",
+
+  t => {
+    const stream = new ReadableStream()
+
+    stub(stream, Symbol.asyncIterator).get(() => undefined)
+
+    t.false(getStreamIterator(stream) instanceof ReadableStream)
+  }
+)
+
+test("Reads from the stream using fallback", async t => {
+  const expected = "Some text"
+
+  const stream = new ReadableStream({
+    pull(controller) {
+      controller.enqueue(new TextEncoder().encode(expected))
+      controller.close()
+    }
+  })
+
+  stub(stream, Symbol.asyncIterator).get(() => undefined)
+
+  let actual = ""
+  const decoder = new TextDecoder()
+  for await (const chunk of getStreamIterator(stream)) {
+    actual += decoder.decode(chunk, {stream: true})
+  }
+
+  actual += decoder.decode()
+
+  t.is(actual, expected)
+})

src/util/getStreamIterator.ts

@@ -0,0 +1,42 @@
+import {isFunction} from "./isFunction.js"
+
+/**
+ * Checks if the value is async iterable
+ */
+const isAsyncIterable = (
+  value: unknown
+): value is AsyncIterable<Uint8Array> => (
+  isFunction((value as AsyncIterable<Uint8Array>)[Symbol.asyncIterator])
+)
+
+/**
+ * Reads from given ReadableStream
+ *
+ * @param readable A ReadableStream to read from
+ */
+async function* readStream(
+  readable: ReadableStream<Uint8Array>
+): AsyncGenerator<Uint8Array, void, undefined> {
+  const reader = readable.getReader()
+
+  while (true) {
+    const {done, value} = await reader.read()
+
+    if (done) {
+      break
+    }
+
+    yield value
+  }
+}
+
+/**
+ * Turns ReadableStream into async iterable when the `Symbol.asyncIterable` is not implemented on given stream.
+ *
+ * @param source A ReadableStream to create async iterator for
+ */
+export const getStreamIterator = (
+  source: ReadableStream<Uint8Array> | AsyncIterable<Uint8Array>
+): AsyncIterable<Uint8Array> => (
+  isAsyncIterable(source) ? source : readStream(source)
+)