add bodymixin.textStream()

lib/web/fetch/body.js

@@ -392,6 +392,49 @@ function bodyMixinMethods (instance, getInternalState) {
       return consumeBody(this, (bytes) => {
         return new Uint8Array(bytes)
       }, instance, getInternalState)
+    },
+
+    textStream () {
+      const this_ = getInternalState(this)
+
+      // 1. If this is unusable, then throw a TypeError.
+      if (bodyUnusable(this_)) {
+        throw new TypeError('Body is unusable: Body has already been read')
+      }
+
+      // 2. If this’s body is null:
+      if (this_.body == null) {
+        // 2.1. Let emptyStream be a new ReadableStream in this’s relevant realm.
+        // 2.2. Set up emptyStream.
+        /** @type {ReadableStreamDefaultController<any>} */
+        let controller
+        const emptyStream = new ReadableStream({
+          start: (c) => {
+            controller = c
+          },
+          pull: () => Promise.resolve(),
+          cancel: () => Promise.resolve()
+        }, {
+          size: () => 1
+        })
+
+        // 2.3. Close emptyStream.
+        controller.close()
+
+        // 2.4. Return emptyStream.
+        return emptyStream
+      }
+
+      // 3. Let stream be this’s body’s stream.
+      /** @type {ReadableStream} */
+      const stream = this_.body.stream
+
+      // 4. Let decoder be a new TextDecoderStream object in this’s relevant realm.
+      // 5. Set up decoder with UTF-8.
+      const decoder = new TextDecoderStream('UTF-8')
+
+      // 6. Return the result of stream, piped through decoder.
+      return stream.pipeThrough(decoder)
     }
   }
 

test/types/fetch.test-d.ts

@@ -177,6 +177,7 @@ expectType<Promise<FormData>>(response.formData())
 expectType<Promise<Uint8Array>>(response.bytes())
 expectType<Promise<unknown>>(response.json())
 expectType<Promise<string>>(response.text())
+expectType<Promise<ReadableStream<string>>>(response.textStream())
 expectType<Response>(response.clone())
 
 expectType<Request>(new Request('https://example.com', { body: 'Hello, world', duplex: 'half' }))

test/web-platform-tests/expectation.json

@@ -6762,6 +6762,67 @@
               "success": true
             }
           ]
+        },
+        "textstream.any.html": {
+          "success": true,
+          "cases": [
+            {
+              "name": "textStream method existence",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() on consumed body throws TypeError",
+              "success": true
+            },
+            {
+              "name": "Request.textStream() on consumed body throws TypeError",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() on locked body throws TypeError",
+              "success": true
+            },
+            {
+              "name": "Request.textStream() on locked body throws TypeError",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() basic functionality",
+              "success": true
+            },
+            {
+              "name": "Request.textStream() basic functionality",
+              "success": true
+            },
+            {
+              "name": "textStream() handles chunked byte stream input",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() with null body",
+              "success": true
+            },
+            {
+              "name": "Request.textStream() with null body",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() with empty body",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() ignores Content-Type charset (UTF-16LE)",
+              "success": true
+            },
+            {
+              "name": "Request.textStream() ignores Content-Type charset (UTF-16LE)",
+              "success": true
+            },
+            {
+              "name": "Response.textStream() ignores invalid Content-Type charset (invalid-charset)",
+              "success": true
+            }
+          ]
         }
       },
       "cors": {

types/fetch.d.ts

@@ -57,6 +57,7 @@ export class BodyMixin {
   readonly formData: () => Promise<FormData>
   readonly json: () => Promise<unknown>
   readonly text: () => Promise<string>
+  readonly textStream: () => Promise<ReadableStream<string>>
 }
 
 export interface SpecIterator<T, TReturn = any, TNext = undefined> {