de-generalize to specific use case and refactor tests

the escaping of this function does is tailored to the specific use case of how bootstrapScriptContent is currently set up and having it be a module suggests it is meant for a more general than it has been considered for.

Additionally the tests were redone to focus on practical implications for what is and is not escaped

packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js

@@ -2811,4 +2811,62 @@ describe('ReactDOMFizzServer', () => {
       </ul>,
     );
   });
+
+  describe('bootstrapScriptContent escaping', () => {
+    it('the "S" in "</?[Ss]cript" strings are replaced with unicode escaped lowercase s or S depending on case, preserving case sensitivity of nearby characters', async () => {
+      window.__test_outlet = '';
+      const stringWithScriptsInIt =
+        'prescription pre<scription pre<Scription pre</scRipTion pre</ScripTion </script><script><!-- <script> -->';
+      await act(async () => {
+        const {pipe} = ReactDOMFizzServer.renderToPipeableStream(<div />, {
+          bootstrapScriptContent:
+            'window.__test_outlet = "This should have been replaced";var x = "' +
+            stringWithScriptsInIt +
+            '";\nwindow.__test_outlet = x;',
+        });
+        pipe(writable);
+      });
+      expect(window.__test_outlet).toMatch(stringWithScriptsInIt);
+    });
+
+    it('does not escape \\u2028, or \\u2029 characters', async () => {
+      // these characters are ignored in engines support https://github.com/tc39/proposal-json-superset
+      // in this test with JSDOM the characters are silently dropped and thus don't need to be encoded.
+      // if you send these characters to an older browser they could fail so it is a good idea to
+      // sanitize JSON input of these characters
+      window.__test_outlet = '';
+      const el = document.createElement('p');
+      el.textContent = '{"one":1,\u2028\u2029"two":2}';
+      const stringWithLSAndPSCharacters = el.textContent;
+      await act(async () => {
+        const {pipe} = ReactDOMFizzServer.renderToPipeableStream(<div />, {
+          bootstrapScriptContent:
+            'let x = ' +
+            stringWithLSAndPSCharacters +
+            '; window.__test_outlet = x;',
+        });
+        pipe(writable);
+      });
+      const outletString = JSON.stringify(window.__test_outlet);
+      expect(outletString).toBe(
+        stringWithLSAndPSCharacters.replace(/[\u2028\u2029]/g, ''),
+      );
+    });
+
+    it('does not escape <, >, or & characters', async () => {
+      // these characters valid javascript and may be necessary in scripts and won't be interpretted properly
+      // escaped outside of a string context within javascript
+      window.__test_outlet = null;
+      // this boolean expression will be cast to a number due to the bitwise &. we will look for a truthy value (1) below
+      const booleanLogicString = '1 < 2 & 3 > 1';
+      await act(async () => {
+        const {pipe} = ReactDOMFizzServer.renderToPipeableStream(<div />, {
+          bootstrapScriptContent:
+            'let x = ' + booleanLogicString + '; window.__test_outlet = x;',
+        });
+        pipe(writable);
+      });
+      expect(window.__test_outlet).toBe(1);
+    });
+  });
 });

packages/react-dom/src/server/ReactDOMServerFormatConfig.js

@@ -52,7 +52,6 @@ import {validateProperties as validateUnknownProperties} from '../shared/ReactDO
 import warnValidStyle from '../shared/warnValidStyle';
 
 import escapeTextForBrowser from './escapeTextForBrowser';
-import escapeScriptForBrowser from './escapeScriptForBrowser';
 import hyphenateStyleName from '../shared/hyphenateStyleName';
 import hasOwnProperty from 'shared/hasOwnProperty';
 import sanitizeURL from '../shared/sanitizeURL';
@@ -84,6 +83,22 @@ const startScriptSrc = stringToPrecomputedChunk('<script src="');
 const startModuleSrc = stringToPrecomputedChunk('<script type="module" src="');
 const endAsyncScript = stringToPrecomputedChunk('" async=""></script>');
 
+const scriptRegex = /(<\/|<)(s)(cript)/gi;
+const substitutions = {
+  s: '\\u0073',
+  S: '\\u0053',
+};
+
+function escapeBootstrapScriptContent(scriptText) {
+  if (__DEV__) {
+    checkHtmlStringCoercion(scriptText);
+  }
+  return ('' + scriptText).replace(
+    scriptRegex,
+    (match, prefix, s, suffix) => `${prefix}${substitutions[s]}${suffix}`,
+  );
+}
+
 // Allows us to keep track of what we've already written so we can refer back to it.
 export function createResponseState(
   identifierPrefix: string | void,
@@ -103,7 +118,7 @@ export function createResponseState(
   if (bootstrapScriptContent !== undefined) {
     bootstrapChunks.push(
       inlineScriptWithNonce,
-      stringToChunk(escapeScriptForBrowser(bootstrapScriptContent)),
+      stringToChunk(escapeBootstrapScriptContent(bootstrapScriptContent)),
       endInlineScript,
     );
   }