Remove "string" from invalid ref error message

String is no longer a valid accepted type. (Except when
`disableStringRefs` is off, in the Meta build, but even in that case
they are deprecated so we shouldn't list it as a valid type.)

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

@@ -414,9 +414,7 @@ describe('ref swapping', () => {
       await act(() => {
         root.render(<div ref={Symbol('foo')} />);
       });
-    }).rejects.toThrow(
-      'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
-    );
+    }).rejects.toThrow('Expected ref to be a function');
   });
 
   // @gate !enableRefAsProp
@@ -434,9 +432,7 @@ describe('ref swapping', () => {
           key: null,
         });
       });
-    }).rejects.toThrow(
-      'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
-    );
+    }).rejects.toThrow('Expected ref to be a function');
   });
 });
 

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -1037,7 +1037,7 @@ function markRef(current: Fiber | null, workInProgress: Fiber) {
     if (typeof ref !== 'function' && typeof ref !== 'object') {
       // TODO: Remove "string" from error message.
       throw new Error(
-        'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
+        'Expected ref to be a function, an object returned by React.createRef(), or undefined/null.',
       );
     }
     if (current === null || current.ref !== ref) {

scripts/error-codes/codes.json

@@ -280,7 +280,7 @@
   "281": "Finished root should have a work-in-progress. This error is likely caused by a bug in React. Please file an issue.",
   "282": "If the root does not have an updateQueue, we should have already bailed out. This error is likely caused by a bug in React. Please file an issue.",
   "283": "Element type is invalid. Received a promise that resolves to: %s. Promise elements must resolve to a class or function.",
-  "284": "Expected ref to be a function, a string, an object returned by React.createRef(), or null.",
+  "284": "Expected ref to be a function, an object returned by React.createRef(), or undefined/null.",
   "285": "The root failed to unmount after an error. This is likely a bug in React. Please file an issue.",
   "286": "%s(...): the first argument must be a React class instance. Instead received: %s.",
   "287": "It is not supported to run the profiling version of a renderer (for example, `react-dom/profiling`) without also replacing the `schedule/tracking` module with `schedule/tracking-profiling`. Your bundler might have a setting for aliasing both modules. Learn more at https://reactjs.org/link/profiling",