Don't crash for undefined globals + tests

Author: rickhanlonii

packages/scheduler/src/__tests__/SchedulerSetImmediate-test.js

@@ -287,3 +287,17 @@ describe('SchedulerDOMSetImmediate', () => {
     runtime.assertLog(['setImmediate Callback', 'B']);
   });
 });
+
+it('does not crash if setImmediate is undefined', () => {
+  jest.resetModules();
+  const originalSetImmediate = global.setImmediate;
+  try {
+    delete global.setImmediate;
+    jest.unmock('scheduler');
+    expect(() => {
+      require('scheduler');
+    }).not.toThrow();
+  } finally {
+    global.setImmediate = originalSetImmediate;
+  }
+});

packages/scheduler/src/__tests__/SchedulerSetTimeout-test.js

@@ -95,3 +95,34 @@ describe('SchedulerNoDOM', () => {
     expect(log).toEqual(['B', 'C']);
   });
 });
+
+// See: https://github.com/facebook/react/pull/13088
+describe('does not crash non-node SSR environments', () => {
+  it('if setTimeout is undefined', () => {
+    jest.resetModules();
+    const originalSetTimeout = global.setTimeout;
+    try {
+      delete global.setTimeout;
+      jest.unmock('scheduler');
+      expect(() => {
+        require('scheduler');
+      }).not.toThrow();
+    } finally {
+      global.setTimeout = originalSetTimeout;
+    }
+  });
+
+  it('if clearTimeout is undefined', () => {
+    jest.resetModules();
+    const originalClearTimeout = global.clearTimeout;
+    try {
+      delete global.clearTimeout;
+      jest.unmock('scheduler');
+      expect(() => {
+        require('scheduler');
+      }).not.toThrow();
+    } finally {
+      global.clearTimeout = originalClearTimeout;
+    }
+  });
+});

packages/scheduler/src/forks/Scheduler.js

@@ -85,8 +85,9 @@ var isHostCallbackScheduled = false;
 var isHostTimeoutScheduled = false;
 
 // Capture local references to native APIs, in case a polyfill overrides them.
-const localSetTimeout = setTimeout;
-const localClearTimeout = clearTimeout;
+const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;
+const localClearTimeout =
+  typeof clearTimeout === 'function' ? clearTimeout : null;
 const localSetImmediate =
   typeof setImmediate !== 'undefined' ? setImmediate : null; // IE and Node.js + jsdom