HtmlUnit · rbri · Oct 30, 2025 · Oct 29, 2025
diff --git a/src/changes/changes.xml b/src/changes/changes.xml
@@ -8,6 +8,9 @@
 
     <body>
         <release version="4.18.0" date="November xx, 2025" description="Chrome/Edge 141, Firefox 144, FirefoxESR 140, Bugfixes">
+            <action type="add" dev="das7pad">
+                Add basic support for parsing multipart/form-data fetch responses
+            </action>
             <action type="fix" dev="rbri">
                 evaluation of the proxy autoconf javascript code fixed (regression)
             </action>

diff --git a/src/main/resources/org/htmlunit/javascript/polyfill/fetch/.editorconfig b/src/main/resources/org/htmlunit/javascript/polyfill/fetch/.editorconfig
@@ -0,0 +1,2 @@
+[fetch.umd.js]
+indent_size = 2
diff --git a/src/main/resources/org/htmlunit/javascript/polyfill/fetch/LICENSE b/src/main/resources/org/htmlunit/javascript/polyfill/fetch/LICENSE
@@ -1,4 +1,5 @@
 Copyright (c) 2014-2016 GitHub, Inc.
+Copyright (c) 2025 Jakob Ackermann <[email protected]>
 
 Permission is hereby granted, free of charge, to any person obtaining
 a copy of this software and associated documentation files (the

diff --git a/src/main/resources/org/htmlunit/javascript/polyfill/fetch/fetch.umd.js b/src/main/resources/org/htmlunit/javascript/polyfill/fetch/fetch.umd.js
@@ -322,7 +322,14 @@
 
     if (support.formData) {
       this.formData = function() {
-        return this.text().then(decode)
+        var body = this;
+        return this.text().then(function (text) {
+          var contentType = body.headers.get('content-type') || '';
+          if (contentType.indexOf('multipart/form-data') === 0) {
+            return parseMultipart(contentType, text);
+          }
+          return decode(text)
+        })
       };
     }
 
@@ -419,6 +426,56 @@
     return form
   }
 
+  /**
+   * @param header
+   * @param parameter
+   * @returns {string | undefined}
+   */
+  function parseHeaderParameter(header, parameter) {
+      var value
+      header.split(';').forEach(function (param) {
+          var keyVal = param.trim().split('=');
+          if (keyVal.length > 1 && keyVal[0] === parameter) {
+              value = keyVal[1];
+              if (value.length > 1 && value[0] === '"' && value[value.length - 1] === '"') {
+                  value = value.slice(1, value.length - 1);
+              }
+          }
+      })
+      return value
+  }
+
+  /**
+   * @param {string} contentType
+   * @param {string} text
+   * @returns {FormData}
+   */
+  function parseMultipart(contentType, text) {
+      var boundary = parseHeaderParameter(contentType, "boundary");
+      if (!boundary) {
+          throw new Error('missing multipart/form-data boundary parameter')
+      }
+      var prefix = '--' + boundary + '\r\n'
+      if (text.indexOf(prefix) !== 0) {
+          throw new Error('multipart/form-data body must start with --boundary')
+      }
+      var suffix = '\r\n--' + boundary + '--'
+      if (text.length < prefix.length + suffix.length || text.slice(text.length - suffix.length) !== suffix) {
+          throw new Error('multipart/form-data body must end with --boundary--')
+      }
+      var formData = new FormData();
+      text.slice(prefix.length, text.length - suffix.length).split('\r\n--' + boundary + '\r\n').forEach(function (part) {
+          var headersEnd = part.indexOf('\r\n\r\n');
+          if (headersEnd === -1) {
+              throw new Error('multipart/form-data part is missing headers')
+          }
+          var headers = parseHeaders(part.slice(0, headersEnd));
+          var name = parseHeaderParameter(headers.get('content-disposition'), 'name');
+          formData.append(name, part.slice(headersEnd + 4))
+      })
+      return formData
+  }
+
   function parseHeaders(rawHeaders) {
     var headers = new Headers();
     // Replace instances of \r\n and \n followed by at least one space or horizontal tab with a space

diff --git a/src/test/java/org/htmlunit/javascript/host/fetch/FetchTest.java b/src/test/java/org/htmlunit/javascript/host/fetch/FetchTest.java
@@ -505,6 +505,45 @@
                 .contains("HtmlUnit"));
     }
 
+    /**
+     * @throws Exception if the test fails
+     */
+    @Test
+    @Disabled
+    @Alerts({"200", "OK", "true"})
+    public void fetchMultipartFormData() throws Exception {
+        final String html = DOCTYPE_HTML
+                + "<html>\n"
+                + "  <body>\n"
+                + "    <script>\n"
+                + LOG_TITLE_FUNCTION_NORMALIZE
+                + "      fetch('" + URL_SECOND + "')"
+                + "        .then(response => {\n"
+                + "          log(response.status);\n"
+                + "          log(response.statusText);\n"
+                + "          log(response.ok);\n"
+                + "         })\n"
+                + "        .then(response => {\n"
+                + "          return response.formData();\n"
+                + "        })\n"
+                + "        .then(formData => {\n"
+                + "            log(formData.get('test0'));\n"
+                + "            log(formData.get('test1'));\n"
+                + "        })\n"
+                + "        .catch(e => logEx(e));\n"
+                + "    </script>\n"
+                + "  </body>\n"
+                + "</html>";
+
+        final String content = "--0123456789\r\nContent-Disposition: form-data;name=test0\r\nContent-Type: text/plain\r\nHello1\nHello1\r\n--0123456789\r\nContent-Disposition: form-data;name=test1\r\nContent-Type: text/plain\r\nHello2\nHello2\r\n--0123456789--";
+        getMockWebConnection().setResponse(URL_SECOND, content, "multipart/form-data; boundary=0123456789");
+
+        final WebDriver driver = loadPage2(html);
+        verifyTitle2(DEFAULT_WAIT_TIME, driver, getExpectedAlerts());
+
+        assertEquals(URL_SECOND, getMockWebConnection().getLastWebRequest().getUrl());
+    }
+
     /**
      * @throws Exception if the test fails
      */