TestRenderer warns if flushThrough is passed the wrong params (facebo…

…ok#12909)

TestRenderer throws if flushThrough is passed the expected yield values that don't match actual yield values.

packages/react-test-renderer/src/ReactTestRendererScheduling.js

@@ -80,7 +80,22 @@ export function flushThrough(expectedValues: Array<mixed>): Array<mixed> {
   }
   if (yieldedValues === null) {
     // Always return an array.
-    return [];
+    yieldedValues = [];
+  }
+  for (let i = 0; i < expectedValues.length; i++) {
+    const expectedValue = `"${(expectedValues[i]: any)}"`;
+    const yieldedValue =
+      i < yieldedValues.length ? `"${(yieldedValues[i]: any)}"` : 'nothing';
+    if (yieldedValue !== expectedValue) {
+      const error = new Error(
+        `flushThrough expected to yield ${(expectedValue: any)}, but ${(yieldedValue: any)} was yielded`,
+      );
+      // Attach expected and yielded arrays,
+      // So the caller could pretty print the diff (if desired).
+      (error: any).expectedValues = expectedValues;
+      (error: any).actualValues = yieldedValues;
+      throw error;
+    }
   }
   return yieldedValues;
 }

packages/react-test-renderer/src/__tests__/ReactTestRendererAsync-test.js

@@ -10,10 +10,16 @@
 
 'use strict';
 
-const React = require('react');
-const ReactTestRenderer = require('react-test-renderer');
+let React;
+let ReactTestRenderer;
 
 describe('ReactTestRendererAsync', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactTestRenderer = require('react-test-renderer');
+  });
+
   it('flushAll flushes all work', () => {
     function Foo(props) {
       return props.children;
@@ -134,4 +140,56 @@ describe('ReactTestRendererAsync', () => {
     // Only the higher priority properties have been committed
     expect(renderer.toJSON()).toEqual(['A:2', 'B:2']);
   });
+
+  it('should error if flushThrough params dont match yielded values', () => {
+    const Yield = ({id}) => {
+      renderer.unstable_yield(id);
+      return id;
+    };
+
+    const renderer = ReactTestRenderer.create(
+      <div>
+        <Yield id="foo" />
+        <Yield id="bar" />
+        <Yield id="baz" />
+      </div>,
+      {
+        unstable_isAsync: true,
+      },
+    );
+
+    expect(() => renderer.unstable_flushThrough(['foo', 'baz'])).toThrow(
+      'flushThrough expected to yield "baz", but "bar" was yielded',
+    );
+  });
+
+  it('should error if flushThrough yields the wrong number of values', () => {
+    const Yield = ({id}) => {
+      renderer.unstable_yield(id);
+      return id;
+    };
+
+    const renderer = ReactTestRenderer.create(
+      <div>
+        <Yield id="foo" />
+      </div>,
+      {
+        unstable_isAsync: true,
+      },
+    );
+
+    expect(() => renderer.unstable_flushThrough(['foo', 'bar'])).toThrow(
+      'flushThrough expected to yield "bar", but nothing was yielded',
+    );
+  });
+
+  it('should error if flushThrough yields no values', () => {
+    const renderer = ReactTestRenderer.create(null, {
+      unstable_isAsync: true,
+    });
+
+    expect(() => renderer.unstable_flushThrough(['foo'])).toThrow(
+      'flushThrough expected to yield "foo", but nothing was yielded',
+    );
+  });
 });