src/net/http/roundtrip_js.go - go - Git at Google

 // Copyright 2018 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 //go:build js && wasm

 package http

 import (
 	"errors"
 	"fmt"
 	"io"
 	"net/http/internal/ascii"
 	"strconv"
 	"strings"
 	"syscall/js"
 )

 var uint8Array = js.Global().Get("Uint8Array")

 // jsFetchMode is a Request.Header map key that, if present,
 // signals that the map entry is actually an option to the Fetch API mode setting.
 // Valid values are: "cors", "no-cors", "same-origin", "navigate"
 // The default is "same-origin".
 //
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchMode = "js.fetch:mode"

 // jsFetchCreds is a Request.Header map key that, if present,
 // signals that the map entry is actually an option to the Fetch API credentials setting.
 // Valid values are: "omit", "same-origin", "include"
 // The default is "same-origin".
 //
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchCreds = "js.fetch:credentials"

 // jsFetchRedirect is a Request.Header map key that, if present,
 // signals that the map entry is actually an option to the Fetch API redirect setting.
 // Valid values are: "follow", "error", "manual"
 // The default is "follow".
 //
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchRedirect = "js.fetch:redirect"

 // jsFetchMissing will be true if the Fetch API is not present in
 // the browser globals.
 var jsFetchMissing = js.Global().Get("fetch").IsUndefined()

 // jsFetchDisabled controls whether the use of Fetch API is disabled.
 // It's set to true when we detect we're running in Node.js, so that
 // RoundTrip ends up talking over the same fake network the HTTP servers
 // currently use in various tests and examples. See go.dev/issue/57613.
 //
 // TODO(go.dev/issue/60810): See if it's viable to test the Fetch API
 // code path.
 var jsFetchDisabled = js.Global().Get("process").Type() == js.TypeObject &&
 	strings.HasPrefix(js.Global().Get("process").Get("argv0").String(), "node")

 // RoundTrip implements the [RoundTripper] interface using the WHATWG Fetch API.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	// The Transport has a documented contract that states that if the DialContext or
 	// DialTLSContext functions are set, they will be used to set up the connections.
 	// If they aren't set then the documented contract is to use Dial or DialTLS, even
 	// though they are deprecated. Therefore, if any of these are set, we should obey
 	// the contract and dial using the regular round-trip instead. Otherwise, we'll try
 	// to fall back on the Fetch API, unless it's not available.
 	if t.Dial != nil || t.DialContext != nil || t.DialTLS != nil || t.DialTLSContext != nil || jsFetchMissing || jsFetchDisabled {
 		return t.roundTrip(req)
 	}

 	ac := js.Global().Get("AbortController")
 	if !ac.IsUndefined() {
 		// Some browsers that support WASM don't necessarily support
 		// the AbortController. See
 		// https://developer.mozilla.org/en-US/docs/Web/API/AbortController#Browser_compatibility.
 		ac = ac.New()
 	}

 	opt := js.Global().Get("Object").New()
 	// See https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch
 	// for options available.
 	opt.Set("method", req.Method)
 	opt.Set("credentials", "same-origin")
 	if h := req.Header.Get(jsFetchCreds); h != "" {
 		opt.Set("credentials", h)
 		req.Header.Del(jsFetchCreds)
 	}
 	if h := req.Header.Get(jsFetchMode); h != "" {
 		opt.Set("mode", h)
 		req.Header.Del(jsFetchMode)
 	}
 	if h := req.Header.Get(jsFetchRedirect); h != "" {
 		opt.Set("redirect", h)
 		req.Header.Del(jsFetchRedirect)
 	}
 	if !ac.IsUndefined() {
 		opt.Set("signal", ac.Get("signal"))
 	}
 	headers := js.Global().Get("Headers").New()
 	for key, values := range req.Header {
 		for _, value := range values {
 			headers.Call("append", key, value)
 		}
 	}
 	opt.Set("headers", headers)

 	if req.Body != nil {
 		// TODO(johanbrandhorst): Stream request body when possible.
 		// See https://bugs.chromium.org/p/chromium/issues/detail?id=688906 for Blink issue.
 		// See https://bugzilla.mozilla.org/show_bug.cgi?id=1387483 for Firefox issue.
 		// See https://github.com/web-platform-tests/wpt/issues/7693 for WHATWG tests issue.
 		// See https://developer.mozilla.org/en-US/docs/Web/API/Streams_API for more details on the Streams API
 		// and browser support.
 		// NOTE(haruyama480): Ensure HTTP/1 fallback exists.
 		// See https://go.dev/issue/61889 for discussion.
 		body, err := io.ReadAll(req.Body)
 		if err != nil {
 			req.Body.Close() // RoundTrip must always close the body, including on errors.
 			return nil, err
 		}
 		req.Body.Close()
 		if len(body) != 0 {
 			buf := uint8Array.New(len(body))
 			js.CopyBytesToJS(buf, body)
 			opt.Set("body", buf)
 		}
 	}

 	fetchPromise := js.Global().Call("fetch", req.URL.String(), opt)
 	var (
 		respCh           = make(chan *Response, 1)
 		errCh            = make(chan error, 1)
 		success, failure js.Func
 	)
 	success = js.FuncOf(func(this js.Value, args []js.Value) any {
 		success.Release()
 		failure.Release()

 		result := args[0]
 		header := Header{}
 		// https://developer.mozilla.org/en-US/docs/Web/API/Headers/entries
 		headersIt := result.Get("headers").Call("entries")
 		for {
 			n := headersIt.Call("next")
 			if n.Get("done").Bool() {
 				break
 			}
 			pair := n.Get("value")
 			key, value := pair.Index(0).String(), pair.Index(1).String()
 			ck := CanonicalHeaderKey(key)
 			header[ck] = append(header[ck], value)
 		}

 		contentLength := int64(0)
 		clHeader := header.Get("Content-Length")
 		switch {
 		case clHeader != "":
 			cl, err := strconv.ParseInt(clHeader, 10, 64)
 			if err != nil {
 				errCh <- fmt.Errorf("net/http: ill-formed Content-Length header: %v", err)
 				return nil
 			}
 			if cl < 0 {
 				// Content-Length values less than 0 are invalid.
 				// See: https://datatracker.ietf.org/doc/html/rfc2616/#section-14.13
 				errCh <- fmt.Errorf("net/http: invalid Content-Length header: %q", clHeader)
 				return nil
 			}
 			contentLength = cl
 		default:
 			// If the response length is not declared, set it to -1.
 			contentLength = -1
 		}

 		b := result.Get("body")
 		var body io.ReadCloser
 		// The body is undefined when the browser does not support streaming response bodies (Firefox),
 		// and null in certain error cases, i.e. when the request is blocked because of CORS settings.
 		if !b.IsUndefined() && !b.IsNull() {
 			body = &streamReader{stream: b.Call("getReader")}
 		} else {
 			// Fall back to using ArrayBuffer
 			// https://developer.mozilla.org/en-US/docs/Web/API/Body/arrayBuffer
 			body = &arrayReader{arrayPromise: result.Call("arrayBuffer")}
 		}

 		code := result.Get("status").Int()

 		uncompressed := false
 		if ascii.EqualFold(header.Get("Content-Encoding"), "gzip") {
 			// The fetch api will decode the gzip, but Content-Encoding not be deleted.
 			header.Del("Content-Encoding")
 			header.Del("Content-Length")
 			contentLength = -1
 			uncompressed = true
 		}

 		respCh <- &Response{
 			Status:        fmt.Sprintf("%d %s", code, StatusText(code)),
 			StatusCode:    code,
 			Header:        header,
 			ContentLength: contentLength,
 			Uncompressed:  uncompressed,
 			Body:          body,
 			Request:       req,
 		}

 		return nil
 	})
 	failure = js.FuncOf(func(this js.Value, args []js.Value) any {
 		success.Release()
 		failure.Release()

 		err := args[0]
 		// The error is a JS Error type
 		// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
 		// We can use the toString() method to get a string representation of the error.
 		errMsg := err.Call("toString").String()
 		// Errors can optionally contain a cause.
 		if cause := err.Get("cause"); !cause.IsUndefined() {
 			// The exact type of the cause is not defined,
 			// but if it's another error, we can call toString() on it too.
 			if !cause.Get("toString").IsUndefined() {
 				errMsg += ": " + cause.Call("toString").String()
 			} else if cause.Type() == js.TypeString {
 				errMsg += ": " + cause.String()
 			}
 		}
 		errCh <- fmt.Errorf("net/http: fetch() failed: %s", errMsg)
 		return nil
 	})

 	fetchPromise.Call("then", success, failure)
 	select {
 	case <-req.Context().Done():
 		if !ac.IsUndefined() {
 			// Abort the Fetch request.
 			ac.Call("abort")
 		}
 		return nil, req.Context().Err()
 	case resp := <-respCh:
 		return resp, nil
 	case err := <-errCh:
 		return nil, err
 	}
 }

 var errClosed = errors.New("net/http: reader is closed")

 // streamReader implements an io.ReadCloser wrapper for ReadableStream.
 // See https://fetch.spec.whatwg.org/#readablestream for more information.
 type streamReader struct {
 	pending []byte
 	stream  js.Value
 	err     error // sticky read error
 }

 func (r *streamReader) Read(p []byte) (n int, err error) {
 	if r.err != nil {
 		return 0, r.err
 	}
 	if len(r.pending) == 0 {
 		var (
 			bCh   = make(chan []byte, 1)
 			errCh = make(chan error, 1)
 		)
 		success := js.FuncOf(func(this js.Value, args []js.Value) any {
 			result := args[0]
 			if result.Get("done").Bool() {
 				errCh <- io.EOF
 				return nil
 			}
 			value := make([]byte, result.Get("value").Get("byteLength").Int())
 			js.CopyBytesToGo(value, result.Get("value"))
 			bCh <- value
 			return nil
 		})
 		defer success.Release()
 		failure := js.FuncOf(func(this js.Value, args []js.Value) any {
 			// Assumes it's a TypeError. See
 			// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError
 			// for more information on this type. See
 			// https://streams.spec.whatwg.org/#byob-reader-read for the spec on
 			// the read method.
 			errCh <- errors.New(args[0].Get("message").String())
 			return nil
 		})
 		defer failure.Release()
 		r.stream.Call("read").Call("then", success, failure)
 		select {
 		case b := <-bCh:
 			r.pending = b
 		case err := <-errCh:
 			r.err = err
 			return 0, err
 		}
 	}
 	n = copy(p, r.pending)
 	r.pending = r.pending[n:]
 	return n, nil
 }

 func (r *streamReader) Close() error {
 	// This ignores any error returned from cancel method. So far, I did not encounter any concrete
 	// situation where reporting the error is meaningful. Most users ignore error from resp.Body.Close().
 	// If there's a need to report error here, it can be implemented and tested when that need comes up.
 	r.stream.Call("cancel")
 	if r.err == nil {
 		r.err = errClosed
 	}
 	return nil
 }

 // arrayReader implements an io.ReadCloser wrapper for ArrayBuffer.
 // https://developer.mozilla.org/en-US/docs/Web/API/Body/arrayBuffer.
 type arrayReader struct {
 	arrayPromise js.Value
 	pending      []byte
 	read         bool
 	err          error // sticky read error
 }

 func (r *arrayReader) Read(p []byte) (n int, err error) {
 	if r.err != nil {
 		return 0, r.err
 	}
 	if !r.read {
 		r.read = true
 		var (
 			bCh   = make(chan []byte, 1)
 			errCh = make(chan error, 1)
 		)
 		success := js.FuncOf(func(this js.Value, args []js.Value) any {
 			// Wrap the input ArrayBuffer with a Uint8Array
 			uint8arrayWrapper := uint8Array.New(args[0])
 			value := make([]byte, uint8arrayWrapper.Get("byteLength").Int())
 			js.CopyBytesToGo(value, uint8arrayWrapper)
 			bCh <- value
 			return nil
 		})
 		defer success.Release()
 		failure := js.FuncOf(func(this js.Value, args []js.Value) any {
 			// Assumes it's a TypeError. See
 			// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError
 			// for more information on this type.
 			// See https://fetch.spec.whatwg.org/#concept-body-consume-body for reasons this might error.
 			errCh <- errors.New(args[0].Get("message").String())
 			return nil
 		})
 		defer failure.Release()
 		r.arrayPromise.Call("then", success, failure)
 		select {
 		case b := <-bCh:
 			r.pending = b
 		case err := <-errCh:
 			return 0, err
 		}
 	}
 	if len(r.pending) == 0 {
 		return 0, io.EOF
 	}
 	n = copy(p, r.pending)
 	r.pending = r.pending[n:]
 	return n, nil
 }

 func (r *arrayReader) Close() error {
 	if r.err == nil {
 		r.err = errClosed
 	}
 	return nil
 }
	// Copyright 2018 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	//go:build js && wasm

	package http

	import (
	"errors"
	"fmt"
	"io"
	"net/http/internal/ascii"
	"strconv"
	"strings"
	"syscall/js"
	)

	var uint8Array = js.Global().Get("Uint8Array")

	// jsFetchMode is a Request.Header map key that, if present,
	// signals that the map entry is actually an option to the Fetch API mode setting.
	// Valid values are: "cors", "no-cors", "same-origin", "navigate"
	// The default is "same-origin".
	//
	// Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
	const jsFetchMode = "js.fetch:mode"

	// jsFetchCreds is a Request.Header map key that, if present,
	// signals that the map entry is actually an option to the Fetch API credentials setting.
	// Valid values are: "omit", "same-origin", "include"
	// The default is "same-origin".
	//
	// Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
	const jsFetchCreds = "js.fetch:credentials"

	// jsFetchRedirect is a Request.Header map key that, if present,
	// signals that the map entry is actually an option to the Fetch API redirect setting.
	// Valid values are: "follow", "error", "manual"
	// The default is "follow".
	//
	// Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
	const jsFetchRedirect = "js.fetch:redirect"

	// jsFetchMissing will be true if the Fetch API is not present in
	// the browser globals.
	var jsFetchMissing = js.Global().Get("fetch").IsUndefined()

	// jsFetchDisabled controls whether the use of Fetch API is disabled.
	// It's set to true when we detect we're running in Node.js, so that
	// RoundTrip ends up talking over the same fake network the HTTP servers
	// currently use in various tests and examples. See go.dev/issue/57613.
	//
	// TODO(go.dev/issue/60810): See if it's viable to test the Fetch API
	// code path.
	var jsFetchDisabled = js.Global().Get("process").Type() == js.TypeObject &&
	strings.HasPrefix(js.Global().Get("process").Get("argv0").String(), "node")

	// RoundTrip implements the [RoundTripper] interface using the WHATWG Fetch API.
	func (t Transport) RoundTrip(req Request) (*Response, error) {
	// The Transport has a documented contract that states that if the DialContext or
	// DialTLSContext functions are set, they will be used to set up the connections.
	// If they aren't set then the documented contract is to use Dial or DialTLS, even
	// though they are deprecated. Therefore, if any of these are set, we should obey
	// the contract and dial using the regular round-trip instead. Otherwise, we'll try
	// to fall back on the Fetch API, unless it's not available.
	if t.Dial != nil \|\| t.DialContext != nil \|\| t.DialTLS != nil \|\| t.DialTLSContext != nil \|\| jsFetchMissing \|\| jsFetchDisabled {
	return t.roundTrip(req)
	}

	ac := js.Global().Get("AbortController")
	if !ac.IsUndefined() {
	// Some browsers that support WASM don't necessarily support
	// the AbortController. See
	// https://developer.mozilla.org/en-US/docs/Web/API/AbortController#Browser_compatibility.
	ac = ac.New()
	}

	opt := js.Global().Get("Object").New()
	// See https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch
	// for options available.
	opt.Set("method", req.Method)
	opt.Set("credentials", "same-origin")
	if h := req.Header.Get(jsFetchCreds); h != "" {
	opt.Set("credentials", h)
	req.Header.Del(jsFetchCreds)
	}
	if h := req.Header.Get(jsFetchMode); h != "" {
	opt.Set("mode", h)
	req.Header.Del(jsFetchMode)
	}
	if h := req.Header.Get(jsFetchRedirect); h != "" {
	opt.Set("redirect", h)
	req.Header.Del(jsFetchRedirect)
	}
	if !ac.IsUndefined() {
	opt.Set("signal", ac.Get("signal"))
	}
	headers := js.Global().Get("Headers").New()
	for key, values := range req.Header {
	for _, value := range values {
	headers.Call("append", key, value)
	}
	}
	opt.Set("headers", headers)

	if req.Body != nil {
	// TODO(johanbrandhorst): Stream request body when possible.
	// See https://bugs.chromium.org/p/chromium/issues/detail?id=688906 for Blink issue.
	// See https://bugzilla.mozilla.org/show_bug.cgi?id=1387483 for Firefox issue.
	// See https://github.com/web-platform-tests/wpt/issues/7693 for WHATWG tests issue.
	// See https://developer.mozilla.org/en-US/docs/Web/API/Streams_API for more details on the Streams API
	// and browser support.
	// NOTE(haruyama480): Ensure HTTP/1 fallback exists.
	// See https://go.dev/issue/61889 for discussion.
	body, err := io.ReadAll(req.Body)
	if err != nil {
	req.Body.Close() // RoundTrip must always close the body, including on errors.
	return nil, err
	}
	req.Body.Close()
	if len(body) != 0 {
	buf := uint8Array.New(len(body))
	js.CopyBytesToJS(buf, body)
	opt.Set("body", buf)
	}
	}

	fetchPromise := js.Global().Call("fetch", req.URL.String(), opt)
	var (
	respCh = make(chan *Response, 1)
	errCh = make(chan error, 1)
	success, failure js.Func
	)
	success = js.FuncOf(func(this js.Value, args []js.Value) any {
	success.Release()
	failure.Release()

	result := args[0]
	header := Header{}
	// https://developer.mozilla.org/en-US/docs/Web/API/Headers/entries
	headersIt := result.Get("headers").Call("entries")
	for {
	n := headersIt.Call("next")
	if n.Get("done").Bool() {
	break
	}
	pair := n.Get("value")
	key, value := pair.Index(0).String(), pair.Index(1).String()
	ck := CanonicalHeaderKey(key)
	header[ck] = append(header[ck], value)
	}

	contentLength := int64(0)
	clHeader := header.Get("Content-Length")
	switch {
	case clHeader != "":
	cl, err := strconv.ParseInt(clHeader, 10, 64)
	if err != nil {
	errCh <- fmt.Errorf("net/http: ill-formed Content-Length header: %v", err)
	return nil
	}
	if cl < 0 {
	// Content-Length values less than 0 are invalid.
	// See: https://datatracker.ietf.org/doc/html/rfc2616/#section-14.13
	errCh <- fmt.Errorf("net/http: invalid Content-Length header: %q", clHeader)
	return nil
	}
	contentLength = cl
	default:
	// If the response length is not declared, set it to -1.
	contentLength = -1
	}

	b := result.Get("body")
	var body io.ReadCloser
	// The body is undefined when the browser does not support streaming response bodies (Firefox),
	// and null in certain error cases, i.e. when the request is blocked because of CORS settings.
	if !b.IsUndefined() && !b.IsNull() {
	body = &streamReader{stream: b.Call("getReader")}
	} else {
	// Fall back to using ArrayBuffer
	// https://developer.mozilla.org/en-US/docs/Web/API/Body/arrayBuffer
	body = &arrayReader{arrayPromise: result.Call("arrayBuffer")}
	}

	code := result.Get("status").Int()

	uncompressed := false
	if ascii.EqualFold(header.Get("Content-Encoding"), "gzip") {
	// The fetch api will decode the gzip, but Content-Encoding not be deleted.
	header.Del("Content-Encoding")
	header.Del("Content-Length")
	contentLength = -1
	uncompressed = true
	}

	respCh <- &Response{
	Status: fmt.Sprintf("%d %s", code, StatusText(code)),
	StatusCode: code,
	Header: header,
	ContentLength: contentLength,
	Uncompressed: uncompressed,
	Body: body,
	Request: req,
	}

	return nil
	})
	failure = js.FuncOf(func(this js.Value, args []js.Value) any {
	success.Release()
	failure.Release()

	err := args[0]
	// The error is a JS Error type
	// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
	// We can use the toString() method to get a string representation of the error.
	errMsg := err.Call("toString").String()
	// Errors can optionally contain a cause.
	if cause := err.Get("cause"); !cause.IsUndefined() {
	// The exact type of the cause is not defined,
	// but if it's another error, we can call toString() on it too.
	if !cause.Get("toString").IsUndefined() {
	errMsg += ": " + cause.Call("toString").String()
	} else if cause.Type() == js.TypeString {
	errMsg += ": " + cause.String()
	}
	}
	errCh <- fmt.Errorf("net/http: fetch() failed: %s", errMsg)
	return nil
	})

	fetchPromise.Call("then", success, failure)
	select {
	case <-req.Context().Done():
	if !ac.IsUndefined() {
	// Abort the Fetch request.
	ac.Call("abort")
	}
	return nil, req.Context().Err()
	case resp := <-respCh:
	return resp, nil
	case err := <-errCh:
	return nil, err
	}
	}

	var errClosed = errors.New("net/http: reader is closed")

	// streamReader implements an io.ReadCloser wrapper for ReadableStream.
	// See https://fetch.spec.whatwg.org/#readablestream for more information.
	type streamReader struct {
	pending []byte
	stream js.Value
	err error // sticky read error
	}

	func (r *streamReader) Read(p []byte) (n int, err error) {
	if r.err != nil {
	return 0, r.err
	}
	if len(r.pending) == 0 {
	var (
	bCh = make(chan []byte, 1)
	errCh = make(chan error, 1)
	)
	success := js.FuncOf(func(this js.Value, args []js.Value) any {
	result := args[0]
	if result.Get("done").Bool() {
	errCh <- io.EOF
	return nil
	}
	value := make([]byte, result.Get("value").Get("byteLength").Int())
	js.CopyBytesToGo(value, result.Get("value"))
	bCh <- value
	return nil
	})
	defer success.Release()
	failure := js.FuncOf(func(this js.Value, args []js.Value) any {
	// Assumes it's a TypeError. See
	// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError
	// for more information on this type. See
	// https://streams.spec.whatwg.org/#byob-reader-read for the spec on
	// the read method.
	errCh <- errors.New(args[0].Get("message").String())
	return nil
	})
	defer failure.Release()
	r.stream.Call("read").Call("then", success, failure)
	select {
	case b := <-bCh:
	r.pending = b
	case err := <-errCh:
	r.err = err
	return 0, err
	}
	}
	n = copy(p, r.pending)
	r.pending = r.pending[n:]
	return n, nil
	}

	func (r *streamReader) Close() error {
	// This ignores any error returned from cancel method. So far, I did not encounter any concrete
	// situation where reporting the error is meaningful. Most users ignore error from resp.Body.Close().
	// If there's a need to report error here, it can be implemented and tested when that need comes up.
	r.stream.Call("cancel")
	if r.err == nil {
	r.err = errClosed
	}
	return nil
	}

	// arrayReader implements an io.ReadCloser wrapper for ArrayBuffer.
	// https://developer.mozilla.org/en-US/docs/Web/API/Body/arrayBuffer.
	type arrayReader struct {
	arrayPromise js.Value
	pending []byte
	read bool
	err error // sticky read error
	}

	func (r *arrayReader) Read(p []byte) (n int, err error) {
	if r.err != nil {
	return 0, r.err
	}
	if !r.read {
	r.read = true
	var (
	bCh = make(chan []byte, 1)
	errCh = make(chan error, 1)
	)
	success := js.FuncOf(func(this js.Value, args []js.Value) any {
	// Wrap the input ArrayBuffer with a Uint8Array
	uint8arrayWrapper := uint8Array.New(args[0])
	value := make([]byte, uint8arrayWrapper.Get("byteLength").Int())
	js.CopyBytesToGo(value, uint8arrayWrapper)
	bCh <- value
	return nil
	})
	defer success.Release()
	failure := js.FuncOf(func(this js.Value, args []js.Value) any {
	// Assumes it's a TypeError. See
	// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypeError
	// for more information on this type.
	// See https://fetch.spec.whatwg.org/#concept-body-consume-body for reasons this might error.
	errCh <- errors.New(args[0].Get("message").String())
	return nil
	})
	defer failure.Release()
	r.arrayPromise.Call("then", success, failure)
	select {
	case b := <-bCh:
	r.pending = b
	case err := <-errCh:
	return 0, err
	}
	}
	if len(r.pending) == 0 {
	return 0, io.EOF
	}
	n = copy(p, r.pending)
	r.pending = r.pending[n:]
	return n, nil
	}

	func (r *arrayReader) Close() error {
	if r.err == nil {
	r.err = errClosed
	}
	return nil
	}