Removed IE9 workaround. Now we assume Node.setAttribute(NS) stringifies.

fixtures/dom/src/components/Header.js

@@ -89,9 +89,6 @@ class Header extends React.Component {
                 <option value="/selection-events">Selection Events</option>
                 <option value="/suspense">Suspense</option>
                 <option value="/form-state">Form State</option>
-                <option value="/attribute-stringification">
-                  Attribute Stringification
-                </option>
               </select>
             </label>
             <label htmlFor="global_version">

fixtures/dom/src/polyfills.js

@@ -2,7 +2,6 @@ import 'core-js/es6/symbol';
 import 'core-js/es6/promise';
 import 'core-js/es6/set';
 import 'core-js/es6/map';
-import 'core-js/features/array/fill';
 
 // http://paulirish.com/2011/requestanimationframe-for-smart-animating/
 // http://my.opera.com/emoller/blog/2011/12/20/requestanimationframe-for-smart-er-animating

packages/react-dom/src/client/DOMPropertyOperations.js

@@ -21,27 +21,6 @@ import {isOpaqueHydratingObject} from './ReactDOMHostConfig';
 
 import type {PropertyInfo} from '../shared/DOMProperty';
 
-/**
- * Cached result of detectStringification() function.
- * Should become true in all environments but IE<=9.
- */
-let setAttributeCanStringify = undefined;
-
-/**
- * Detect if Element.setAttribute stringifies attribute values.
- * Should return true for all environments but IE <= 9.
- * @param {DOMElement} node
- */
-function detectStringification(node: Element) {
-  const obj: any = {
-    toString: () => 'foo',
-  };
-  const attrName = 'title';
-  const el = node.ownerDocument.createElement('p');
-  el.setAttribute(attrName, obj);
-  return el.getAttribute(attrName) === 'foo';
-}
-
 /**
  * Get the value for a property on a node. Only used in DEV for SSR validation.
  * The "expected" argument is used as a hint of what the expected value is.
@@ -164,19 +143,17 @@ export function setValueForProperty(
   if (shouldRemoveAttribute(name, value, propertyInfo, isCustomComponentTag)) {
     value = null;
   }
-  if (setAttributeCanStringify === undefined) {
-    setAttributeCanStringify = detectStringification(node);
-  }
   // If the prop isn't in the special list, treat it as a simple attribute.
   if (isCustomComponentTag || propertyInfo === null) {
     if (isAttributeNameSafe(name)) {
       const attributeName = name;
       if (value === null) {
         node.removeAttribute(attributeName);
       } else {
+        // Node.setAttribute(NS) stringifies the value implicitly in all browsers apart from IE <= 9.
         node.setAttribute(
           attributeName,
-          setAttributeCanStringify ? (value: any) : '' + (value: any),
+          (value: any),
         );
       }
     }
@@ -207,13 +184,8 @@ export function setValueForProperty(
       // and we won't require Trusted Type here.
       attributeValue = '';
     } else {
-      if (setAttributeCanStringify) {
-        attributeValue = (value: any);
-      } else {
-        // As `setAttribute` does not stringify the value itself.
-        // ('' + value) makes it output the correct toString()-value.
-        attributeValue = '' + (value: any);
-      }
+      // Node.setAttribute(NS) stringifies the value implicitly in all browsers apart from IE <= 9.
+      attributeValue = (value: any);
       if (propertyInfo.sanitizeURL) {
         attributeValue = sanitizeURL(attributeValue);
       }

packages/react-dom/src/client/__tests__/trustedTypes-test.internal.js

@@ -61,9 +61,6 @@ describe('when Trusted Types are available in global object', () => {
     };
     fakeTTObjects.add(ttObject1);
     fakeTTObjects.add(ttObject2);
-    // Run setAttributeCanStringify detection first to simplify counting
-    // setAttribute calls later.
-    ReactDOM.render(<div foo="bar" />, container);
   });
 
   afterEach(() => {