refactor(techdocs): create html sanitizer transformer

Signed-off-by: Camila Belo <camilaibs@gmail.com>

plugins/techdocs/src/reader/transformers/html/transformer.ts

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2022 The Backstage Authors
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+import DOMPurify from 'dompurify';
+import { useMemo, useCallback } from 'react';
+
+import { useApi, configApiRef } from '@backstage/core-plugin-api';
+
+import { Transformer } from '..';
+import { removeUnsafeLinks, removeUnsafeIframes } from './hooks';
+
+/**
+ * Returns html sanitizer configuration
+ */
+const useSanitizerConfig = () => {
+  const configApi = useApi(configApiRef);
+
+  return useMemo(() => {
+    return configApi.getOptionalConfig('techdocs.sanitizer');
+  }, [configApi]);
+};
+
+/**
+ * Returns a transformer that sanitizes the dom's internal html.
+ */
+export const useHtmlTransformer = (): Transformer => {
+  const config = useSanitizerConfig();
+
+  return useCallback(
+    async (dom: Element) => {
+      const hosts = config?.getOptionalStringArray('allowedIframeHosts');
+
+      DOMPurify.addHook('beforeSanitizeElements', removeUnsafeLinks);
+      const tags = ['link'];
+
+      if (hosts) {
+        tags.push('iframe');
+        DOMPurify.addHook('beforeSanitizeElements', removeUnsafeIframes(hosts));
+      }
+
+      return DOMPurify.sanitize(dom.innerHTML, {
+        ADD_TAGS: tags,
+        FORBID_TAGS: ['style'],
+        WHOLE_DOCUMENT: true,
+        RETURN_DOM: true,
+      });
+    },
+    [config],
+  );
+};