Implement native Fetch API host support (Window.fetch, Request/Response/Headers, body mixin)

src/main/java/org/htmlunit/javascript/JavaScriptEngine.java

@@ -82,6 +82,7 @@
 import org.htmlunit.javascript.host.Window;
 import org.htmlunit.javascript.host.WindowOrWorkerGlobalScope;
 import org.htmlunit.javascript.host.dom.DOMException;
+import org.htmlunit.javascript.host.fetch.Headers;
 import org.htmlunit.javascript.host.html.HTMLElement;
 import org.htmlunit.javascript.host.html.HTMLImageElement;
 import org.htmlunit.javascript.host.html.HTMLOptionElement;
@@ -248,6 +249,7 @@ private void init(final WebWindow webWindow, final Page page, final Context cx)
         // TODO remove the cast
         URLSearchParams.NativeParamsIterator.init(scope, "URLSearchParams Iterator");
         FormData.FormDataIterator.init(scope, "FormData Iterator");
+        Headers.NativeHeadersIterator.init(scope, "Headers Iterator");
 
         // strange but this is the reality for browsers
         // because there will be still some sites using this for browser detection the property is

src/main/java/org/htmlunit/javascript/host/Window.java

@@ -52,6 +52,8 @@
 import org.htmlunit.TopLevelWindow;
 import org.htmlunit.WebAssert;
 import org.htmlunit.WebClient;
+import org.htmlunit.WebRequest;
+import org.htmlunit.WebResponse;
 import org.htmlunit.WebConsole;
 import org.htmlunit.WebWindow;
 import org.htmlunit.WebWindowNotFoundException;
@@ -61,6 +63,7 @@
 import org.htmlunit.corejs.javascript.Function;
 import org.htmlunit.corejs.javascript.JavaScriptException;
 import org.htmlunit.corejs.javascript.NativeConsole.Level;
+import org.htmlunit.corejs.javascript.NativePromise;
 import org.htmlunit.corejs.javascript.NativeObject;
 import org.htmlunit.corejs.javascript.Scriptable;
 import org.htmlunit.corejs.javascript.ScriptableObject;
@@ -110,6 +113,8 @@
 import org.htmlunit.javascript.host.event.MessageEvent;
 import org.htmlunit.javascript.host.event.MouseEvent;
 import org.htmlunit.javascript.host.event.UIEvent;
+import org.htmlunit.javascript.host.fetch.Request;
+import org.htmlunit.javascript.host.fetch.Response;
 import org.htmlunit.javascript.host.html.DocumentProxy;
 import org.htmlunit.javascript.host.html.HTMLCollection;
 import org.htmlunit.javascript.host.html.HTMLDocument;
@@ -304,6 +309,40 @@ public String atob(final String encodedData) {
         return WindowOrWorkerGlobalScopeMixin.atob(encodedData, this);
     }
 
+    /**
+     * The JavaScript function {@code fetch()}.
+     * @param input a string or a Request
+     * @param init optional init dictionary
+     * @return a promise resolving to the response
+     */
+    @JsxFunction
+    public NativePromise fetch(final Object input, final Object init) {
+        try {
+            final VarScope scope = JavaScriptEngine.getTopCallScope();
+            final Request request =
+                    (Request) JavaScriptEngine.newObject(scope, "Request", new Object[] {input, init});
+            final WebRequest webRequest = request.toWebRequest(this);
+            final WebResponse webResponse = getWebWindow().getWebClient().loadWebResponse(webRequest);
+
+            final Response response = (Response) JavaScriptEngine.newObject(scope,
+                    "Response", new Object[] {JavaScriptEngine.UNDEFINED, JavaScriptEngine.UNDEFINED});
+            response.setFromWebResponse(webResponse, webRequest.getUrl().toExternalForm());
+            return setupPromise(() -> response);
+        }
+        catch (final Exception e) {
+            return setupRejectedPromise(() -> {
+                final VarScope scope = JavaScriptEngine.getTopCallScope();
+                final Object typeError = ScriptableObject.getProperty(scope, "TypeError");
+                if (typeError instanceof Function function) {
+                    final String message = org.htmlunit.util.StringUtils.isEmptyOrNull(e.getMessage())
+                            ? "Failed to fetch" : e.getMessage();
+                    return function.construct(Context.getCurrentContext(), scope, new Object[] {message});
+                }
+                return e.getMessage();
+            });
+        }
+    }
+
     /**
      * The JavaScript function {@code confirm}.
      * @param message the message

src/main/java/org/htmlunit/javascript/host/fetch/FetchBodyMixin.java

@@ -0,0 +1,210 @@
+/*
+ * Copyright (c) 2002-2026 Gargoyle Software Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.htmlunit.javascript.host.fetch;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import java.io.IOException;
+
+import org.htmlunit.corejs.javascript.Context;
+import org.htmlunit.corejs.javascript.Function;
+import org.htmlunit.corejs.javascript.NativeArrayBuffer;
+import org.htmlunit.corejs.javascript.NativePromise;
+import org.htmlunit.corejs.javascript.ScriptableObject;
+import org.htmlunit.corejs.javascript.VarScope;
+import org.htmlunit.corejs.javascript.json.JsonParser;
+import org.htmlunit.corejs.javascript.json.JsonParser.ParseException;
+import org.htmlunit.javascript.HtmlUnitScriptable;
+import org.htmlunit.javascript.JavaScriptEngine;
+import org.htmlunit.javascript.configuration.JsxFunction;
+import org.htmlunit.javascript.configuration.JsxGetter;
+import org.htmlunit.javascript.host.file.Blob;
+import org.htmlunit.javascript.host.xml.FormData;
+
+/**
+ * Shared body handling for fetch Request/Response objects.
+ *
+ * @author Ronald Brill
+ */
+public class FetchBodyMixin extends HtmlUnitScriptable {
+
+    private byte[] bodyBytes_ = new byte[0];
+    private String contentType_;
+    private boolean bodyUsed_;
+
+    @JsxGetter
+    public Object getBody() {
+        return null;
+    }
+
+    @JsxGetter
+    public boolean isBodyUsed() {
+        return bodyUsed_;
+    }
+
+    @JsxFunction
+    public NativePromise text() {
+        if (bodyUsed_) {
+            return rejectedTypeErrorPromise("Body has already been consumed.");
+        }
+        bodyUsed_ = true;
+        return setupPromise(() -> new String(bodyBytes_, UTF_8));
+    }
+
+    @JsxFunction
+    public NativePromise json() {
+        if (bodyUsed_) {
+            return rejectedTypeErrorPromise("Body has already been consumed.");
+        }
+        bodyUsed_ = true;
+        try {
+            final String json = new String(bodyBytes_, UTF_8);
+            final Object parsed = new JsonParser(Context.getCurrentContext(), getParentScope()).parseValue(json);
+            return setupPromise(() -> parsed);
+        }
+        catch (final ParseException e) {
+            return rejectedTypeErrorPromise(e.getMessage());
+        }
+    }
+
+    @JsxFunction
+    public NativePromise arrayBuffer() {
+        if (bodyUsed_) {
+            return rejectedTypeErrorPromise("Body has already been consumed.");
+        }
+        bodyUsed_ = true;
+        return setupPromise(() -> {
+            final NativeArrayBuffer buffer = new NativeArrayBuffer(bodyBytes_.length);
+            System.arraycopy(bodyBytes_, 0, buffer.getBuffer(), 0, bodyBytes_.length);
+            buffer.setParentScope(getParentScope());
+            buffer.setPrototype(ScriptableObject.getClassPrototype(getParentScope(), buffer.getClassName()));
+            return buffer;
+        });
+    }
+
+    @JsxFunction
+    public NativePromise blob() {
+        if (bodyUsed_) {
+            return rejectedTypeErrorPromise("Body has already been consumed.");
+        }
+        bodyUsed_ = true;
+        return setupPromise(() -> {
+            final Blob blob = new Blob(bodyBytes_, contentType_);
+            blob.setParentScope(getParentScope());
+            blob.setPrototype(getPrototype(Blob.class));
+            return blob;
+        });
+    }
+
+    @JsxFunction
+    public NativePromise formData() {
+        if (bodyUsed_) {
+            return rejectedTypeErrorPromise("Body has already been consumed.");
+        }
+        bodyUsed_ = true;
+        return setupPromise(this::buildFormData);
+    }
+
+    protected byte[] getBodyBytes() {
+        return bodyBytes_;
+    }
+
+    protected String getBodyContentType() {
+        return contentType_;
+    }
+
+    protected void setBody(final byte[] bodyBytes, final String contentType) {
+        bodyBytes_ = bodyBytes != null ? bodyBytes.clone() : new byte[0];
+        contentType_ = contentType;
+        bodyUsed_ = false;
+    }
+
+    protected void copyBodyTo(final FetchBodyMixin other) {
+        other.bodyBytes_ = bodyBytes_.clone();
+        other.contentType_ = contentType_;
+        other.bodyUsed_ = false;
+    }
+
+    protected Object createTypeErrorObject(final String message) {
+        final VarScope scope = ScriptableObject.getTopLevelScope(getParentScope());
+        final Object typeError = ScriptableObject.getProperty(scope, "TypeError");
+        if (typeError instanceof Function function) {
+            return function.construct(Context.getCurrentContext(), scope, new Object[] {message});
+        }
+        return message;
+    }
+
+    protected NativePromise rejectedTypeErrorPromise(final String message) {
+        return setupRejectedPromise(() -> createTypeErrorObject(message));
+    }
+
+    private FormData buildFormData() throws IOException {
+        final FormData formData = new FormData();
+        formData.setParentScope(getParentScope());
+        formData.setPrototype(getPrototype(FormData.class));
+        formData.jsConstructor(JavaScriptEngine.UNDEFINED);
+
+        final String contentType = contentType_ != null ? contentType_.toLowerCase() : "";
+        if (contentType.startsWith("multipart/form-data;")) {
+            final String marker = "boundary=";
+            final int boundaryStart = contentType.indexOf(marker);
+            if (boundaryStart > -1) {
+                final String boundary = contentType.substring(boundaryStart + marker.length()).trim();
+                final String body = new String(bodyBytes_, UTF_8);
+                final String[] parts = body.split("--" + boundary);
+                for (final String part : parts) {
+                    final int headerEnd = part.indexOf("\r\n\r\n");
+                    if (headerEnd < 0) {
+                        continue;
+                    }
+                    final String headers = part.substring(0, headerEnd);
+                    final String data = part.substring(headerEnd + 4).replaceFirst("\r\n$", "");
+                    final String name = extractName(headers);
+                    if (name != null) {
+                        formData.append(name, data, JavaScriptEngine.UNDEFINED);
+                    }
+                }
+                return formData;
+            }
+        }
+
+        final String body = new String(bodyBytes_, UTF_8);
+        if (!body.isEmpty()) {
+            final String[] pairs = body.split("&");
+            for (final String pair : pairs) {
+                final int equals = pair.indexOf('=');
+                final String name = equals < 0 ? pair : pair.substring(0, equals);
+                final String value = equals < 0 ? "" : pair.substring(equals + 1);
+                formData.append(org.htmlunit.util.UrlUtils.decode(name),
+                        org.htmlunit.util.UrlUtils.decode(value), JavaScriptEngine.UNDEFINED);
+            }
+        }
+
+        return formData;
+    }
+
+    private static String extractName(final String headers) {
+        final String marker = "name=\"";
+        final int start = headers.indexOf(marker);
+        if (start < 0) {
+            return null;
+        }
+        final int end = headers.indexOf('"', start + marker.length());
+        if (end < 0) {
+            return null;
+        }
+        return headers.substring(start + marker.length(), end);
+    }
+}