feat: toHaveStyle matcher

package.json

@@ -82,6 +82,7 @@
     "flow": "flow",
     "copy-flowtypes": "cp typings/index.flow.js build",
     "lint": "eslint src --cache",
+    "validate": "yarn lint && yarn typecheck && yarn test",
     "prepublish": "yarn build",
     "build:js": "babel src --out-dir build --extensions \".js,.ts,.jsx,.tsx\" --source-maps --ignore \"**/__tests__/**\"",
     "build:js:watch": "yarn build:js --watch",

src/matchers/__tests__/to-have-style.test.tsx

@@ -0,0 +1,175 @@
+import React from 'react';
+import { StyleSheet, View, Pressable } from 'react-native';
+import { render } from '../..';
+import '../extend-expect';
+
+const styles = StyleSheet.create({
+  container: { borderBottomColor: 'white' },
+});
+
+test('toHaveStyle() handles basic cases', () => {
+  const screen = render(
+    <View
+      testID="view"
+      style={[
+        {
+          backgroundColor: 'blue',
+          height: '40%',
+          transform: [{ scale: 2 }, { rotate: '45deg' }],
+        },
+        [{ height: '100%' }],
+        [[{ width: '50%' }]],
+        styles.container,
+      ]}
+    />
+  );
+
+  const view = screen.getByTestId('view');
+  expect(view).toHaveStyle({ backgroundColor: 'blue' });
+  expect(view).toHaveStyle({ height: '100%' });
+  expect(view).toHaveStyle({ backgroundColor: 'blue', height: '100%' });
+  expect(view).toHaveStyle([{ backgroundColor: 'blue' }, { height: '100%' }]);
+
+  expect(view).toHaveStyle({ borderBottomColor: 'white' });
+  expect(view).toHaveStyle({ width: '50%' });
+  expect(view).toHaveStyle([[{ width: '50%' }]]);
+  expect(view).toHaveStyle({
+    transform: [{ scale: 2 }, { rotate: '45deg' }],
+  });
+
+  expect(view).not.toHaveStyle({ backgroundColor: 'red' });
+  expect(view).not.toHaveStyle({ height: '50%' });
+  expect(view).not.toHaveStyle({ backgroundColor: 'blue', height: '50%' });
+  expect(view).not.toHaveStyle([
+    { backgroundColor: 'blue' },
+    { height: '50%' },
+  ]);
+  expect(view).not.toHaveStyle({
+    transform: [{ scale: 2 }],
+  });
+  expect(view).not.toHaveStyle({
+    transform: [{ rotate: '45deg' }, { scale: 2 }],
+  });
+});
+
+test('toHaveStyle error messages', () => {
+  const screen = render(
+    <View
+      testID="view"
+      style={{
+        backgroundColor: 'blue',
+        borderBottomColor: 'black',
+        height: '100%',
+        transform: [{ scale: 2 }, { rotate: '45deg' }],
+      }}
+    />
+  );
+
+  const view = screen.getByTestId('view');
+  expect(() => expect(view).toHaveStyle({ backgroundColor: 'red' }))
+    .toThrowErrorMatchingInlineSnapshot(`
+    "expect(element).toHaveStyle()
+
+    - Expected
+    + Received
+
+    - backgroundColor: red;
+    + backgroundColor: blue;"
+  `);
+
+  expect(() =>
+    expect(view).toHaveStyle({
+      backgroundColor: 'blue',
+      transform: [{ scale: 1 }],
+    })
+  ).toThrowErrorMatchingInlineSnapshot(`
+    "expect(element).toHaveStyle()
+
+    - Expected
+    + Received
+
+      backgroundColor: blue;
+      transform: [
+        {
+    -     "scale": 1
+    +     "scale": 2
+    +   },
+    +   {
+    +     "rotate": "45deg"
+        }
+      ];"
+  `);
+
+  expect(() => expect(view).not.toHaveStyle({ backgroundColor: 'blue' }))
+    .toThrowErrorMatchingInlineSnapshot(`
+    "expect(element).not.toHaveStyle()
+
+    Expected element not to have style:
+      backgroundColor: blue;
+    Received:
+      backgroundColor: blue;"
+  `);
+
+  expect(() => expect(view).toHaveStyle({ fontWeight: 'bold' }))
+    .toThrowErrorMatchingInlineSnapshot(`
+    "expect(element).toHaveStyle()
+
+    - Expected
+    + Received
+
+    - fontWeight: bold;"
+  `);
+
+  expect(() => expect(view).not.toHaveStyle({ backgroundColor: 'blue' }))
+    .toThrowErrorMatchingInlineSnapshot(`
+    "expect(element).not.toHaveStyle()
+
+    Expected element not to have style:
+      backgroundColor: blue;
+    Received:
+      backgroundColor: blue;"
+  `);
+});
+
+test('toHaveStyle() supports missing "style" prop', () => {
+  const screen = render(<View testID="view" />);
+
+  const view = screen.getByTestId('view');
+  expect(view).not.toHaveStyle({ fontWeight: 'bold' });
+});
+
+test('toHaveStyle() supports undefined "transform" style', () => {
+  const screen = render(
+    <View
+      testID="view"
+      style={{
+        backgroundColor: 'blue',
+        transform: undefined,
+      }}
+    />
+  );
+
+  const view = screen.getByTestId('view');
+  expect(() => expect(view).toHaveStyle({ transform: [{ scale: 1 }] }))
+    .toThrowErrorMatchingInlineSnapshot(`
+    "expect(element).toHaveStyle()
+
+    - Expected
+    + Received
+
+    - transform: [
+    -   {
+    -     "scale": 1
+    -   }
+    - ];
+    + transform: undefined;"
+  `);
+});
+
+test('toHaveStyle() supports Pressable with function "style" prop', () => {
+  const screen = render(
+    <Pressable testID="view" style={() => ({ backgroundColor: 'blue' })} />
+  );
+
+  expect(screen.getByTestId('view')).toHaveStyle({ backgroundColor: 'blue' });
+});

src/matchers/extend-expect.d.ts

@@ -1,4 +1,6 @@
+import type { StyleProp } from 'react-native';
 import type { TextMatch, TextMatchOptions } from '../matches';
+import type { Style } from './to-have-style';
 
 export interface JestNativeMatchers<R> {
   toBeOnTheScreen(): R;
@@ -12,6 +14,7 @@ export interface JestNativeMatchers<R> {
   toBeVisible(): R;
   toHaveDisplayValue(expectedValue: TextMatch, options?: TextMatchOptions): R;
   toHaveProp(name: string, expectedValue?: unknown): R;
+  toHaveStyle(style: StyleProp<Style>): R;
   toHaveTextContent(expectedText: TextMatch, options?: TextMatchOptions): R;
 }
 

src/matchers/extend-expect.ts

@@ -10,6 +10,7 @@ import { toBeSelected } from './to-be-selected';
 import { toBeVisible } from './to-be-visible';
 import { toHaveDisplayValue } from './to-have-display-value';
 import { toHaveProp } from './to-have-prop';
+import { toHaveStyle } from './to-have-style';
 import { toHaveTextContent } from './to-have-text-content';
 
 expect.extend({
@@ -24,5 +25,6 @@ expect.extend({
   toBeVisible,
   toHaveDisplayValue,
   toHaveProp,
+  toHaveStyle,
   toHaveTextContent,
 });

src/matchers/index.tsx

@@ -7,5 +7,6 @@ export { toBePartiallyChecked } from './to-be-partially-checked';
 export { toBeVisible } from './to-be-visible';
 export { toHaveDisplayValue } from './to-have-display-value';
 export { toHaveProp } from './to-have-prop';
+export { toHaveStyle } from './to-have-style';
 export { toHaveTextContent } from './to-have-text-content';
 export { toBeSelected } from './to-be-selected';

src/matchers/to-have-style.tsx

@@ -0,0 +1,86 @@
+import type { ReactTestInstance } from 'react-test-renderer';
+import {
+  ImageStyle,
+  StyleProp,
+  StyleSheet,
+  TextStyle,
+  ViewStyle,
+} from 'react-native';
+import { matcherHint, diff } from 'jest-matcher-utils';
+import { checkHostElement, formatMessage } from './utils';
+
+export type Style = ViewStyle | TextStyle | ImageStyle;
+
+type StyleLike = Record<string, unknown>;
+
+export function toHaveStyle(
+  this: jest.MatcherContext,
+  element: ReactTestInstance,
+  style: StyleProp<Style>
+) {
+  checkHostElement(element, toHaveStyle, this);
+
+  const expected = (StyleSheet.flatten(style) as StyleLike) ?? {};
+  const received = (StyleSheet.flatten(element.props.style) as StyleLike) ?? {};
+
+  const pass = Object.keys(expected).every((key) =>
+    this.equals(expected[key], received[key])
+  );
+
+  return {
+    pass,
+    message: () => {
+      const to = this.isNot ? 'not to' : 'to';
+      const matcher = matcherHint(
+        `${this.isNot ? '.not' : ''}.toHaveStyle`,
+        'element',
+        ''
+      );
+
+      if (pass) {
+        return formatMessage(
+          matcher,
+          `Expected element ${to} have style`,
+          formatStyles(expected),
+          'Received',
+          formatStyles(pickReceivedStyles(expected, received))
+        );
+      } else {
+        return [matcher, '', expectedDiff(expected, received)].join('\n');
+      }
+    },
+  };
+}
+
+/**
+ * Generate diff between `expected` and `received` styles.
+ */
+function expectedDiff(expected: StyleLike, received: StyleLike) {
+  const receivedNarrow = pickReceivedStyles(expected, received);
+  return diff(formatStyles(expected), formatStyles(receivedNarrow));
+}
+
+/**
+ * Pick from `received` style only the keys present in `expected` style.
+ */
+function pickReceivedStyles(expected: StyleLike, received: StyleLike) {
+  const result: StyleLike = {};
+  Object.keys(received).forEach((key) => {
+    if (expected[key] !== undefined) {
+      result[key] = received[key];
+    }
+  });
+
+  return result;
+}
+
+function formatStyles(style: StyleLike) {
+  return Object.keys(style)
+    .sort()
+    .map((prop) =>
+      Array.isArray(style[prop])
+        ? `${prop}: ${JSON.stringify(style[prop], null, 2)};`
+        : `${prop}: ${style[prop]};`
+    )
+    .join('\n');
+}