feature: toHaveStyle custom matcher (#12)

* feature: toHaveStyle custom matcher

* Fix test coverage

* Use more robust css parser library

* Handle css parsing errors gracefully

* Improve how printed out styles look like in failing tests messages

* Add documentation to the README

* Use redent for indent

README.md

@@ -46,6 +46,7 @@ to maintain.
   * [`toHaveTextContent`](#tohavetextcontent)
   * [`toHaveAttribute`](#tohaveattribute)
   * [`toHaveClass`](#tohaveclass)
+  * [`toHaveStyle`](#tohavestyle)
 * [Inspiration](#inspiration)
 * [Other Solutions](#other-solutions)
 * [Guiding Principles](#guiding-principles)
@@ -163,6 +164,36 @@ expect(getByTestId(container, 'delete-button')).not.toHaveClass('btn-link')
 // ...
 ```
 
+### `toHaveStyle`
+
+This allows you to check if a certain element has some specific css properties
+with specific values applied. It matches only if the element has _all_ the
+expected properties applied, not just some of them.
+
+```javascript
+// add the custom expect matchers once
+import 'jest-dom/extend-expect'
+
+// ...
+// <button data-testid="delete-button" style="display: none; color: red">
+//   Delete item
+// </button>
+expect(getByTestId(container, 'delete-button')).toHaveStyle('display: none')
+expect(getByTestId(container, 'delete-button')).toHaveStyle(`
+  color: red;
+  display: none;
+`)
+expect(getByTestId(container, 'delete-button')).not.toHaveStyle(`
+  display: none;
+  color: blue;
+`)
+// ...
+```
+
+This also works with rules that are applied to the element via a class name for
+which some rules are defined in a stylesheet currently active in the document.
+The usual rules of css precedence apply.
+
 ## Inspiration
 
 This whole library was extracted out of Kent C. Dodds' [dom-testing-library][],

extend-expect.d.ts

@@ -4,5 +4,6 @@ declare namespace jest {
     toHaveTextContent: (text: string) => R
     toHaveClass: (className: string) => R
     toBeInTheDOM: () => R
+    toHaveStyle: (css: string) => R
   }
 }

package.json

@@ -32,7 +32,11 @@
   "author": "Ernesto Garcia <[email protected]> (http://gnapse.github.io/)",
   "license": "MIT",
   "dependencies": {
-    "jest-matcher-utils": "^22.4.3"
+    "chalk": "^2.4.1",
+    "css": "^2.2.3",
+    "jest-diff": "^22.4.3",
+    "jest-matcher-utils": "^22.4.3",
+    "redent": "^2.0.0"
   },
   "devDependencies": {
     "kcd-scripts": "^0.37.0"

src/__tests__/index.js

@@ -109,3 +109,61 @@ test('.toHaveClass', () => {
     expect(queryByTestId('cancel-button')).toHaveClass('btn-danger'),
   ).toThrowError()
 })
+
+test('.toHaveStyle', () => {
+  const {container} = render(`
+    <div class="label" style="background-color: blue; height: 100%">
+      Hello World
+    </div>
+  `)
+
+  const style = document.createElement('style')
+  style.innerHTML = `
+    .label {
+      background-color: black;
+      color: white;
+      float: left;
+    }
+  `
+  document.body.appendChild(style)
+  document.body.appendChild(container)
+
+  expect(container.querySelector('.label')).toHaveStyle(`
+    height: 100%;
+    color: white;
+    background-color: blue;
+  `)
+
+  expect(container.querySelector('.label')).toHaveStyle(`
+    background-color: blue;
+    color: white;
+  `)
+  expect(container.querySelector('.label')).toHaveStyle(
+    'background-color:blue;color:white',
+  )
+
+  expect(container.querySelector('.label')).not.toHaveStyle(`
+    color: white;
+    font-weight: bold;
+  `)
+
+  expect(() =>
+    expect(container.querySelector('.label')).toHaveStyle('font-weight: bold'),
+  ).toThrowError()
+  expect(() =>
+    expect(container.querySelector('.label')).not.toHaveStyle('color: white'),
+  ).toThrowError()
+
+  // Make sure the test fails if the css syntax is not valid
+  expect(() =>
+    expect(container.querySelector('.label')).not.toHaveStyle(
+      'font-weight bold',
+    ),
+  ).toThrowError()
+  expect(() =>
+    expect(container.querySelector('.label')).toHaveStyle('color white'),
+  ).toThrowError()
+
+  document.body.removeChild(style)
+  document.body.removeChild(container)
+})

src/index.js

@@ -2,5 +2,12 @@ import {toBeInTheDOM} from './to-be-in-the-dom'
 import {toHaveTextContent} from './to-have-text-content'
 import {toHaveAttribute} from './to-have-attribute'
 import {toHaveClass} from './to-have-class'
+import {toHaveStyle} from './to-have-style'
 
-export {toBeInTheDOM, toHaveTextContent, toHaveAttribute, toHaveClass}
+export {
+  toBeInTheDOM,
+  toHaveTextContent,
+  toHaveAttribute,
+  toHaveClass,
+  toHaveStyle,
+}

src/to-have-style.js

@@ -0,0 +1,75 @@
+import {parse} from 'css'
+import {matcherHint} from 'jest-matcher-utils'
+import jestDiff from 'jest-diff'
+import chalk from 'chalk'
+import {checkHtmlElement} from './utils'
+
+function parseCSS(css) {
+  const ast = parse(`selector { ${css} }`, {silent: true}).stylesheet
+  if (ast.parsingErrors && ast.parsingErrors.length > 0) {
+    const {reason, line, column} = ast.parsingErrors[0]
+    return {
+      parsingError: `Syntax error parsing expected css: ${reason} in ${line}:${column}`,
+    }
+  }
+  const parsedRules = ast.rules[0].declarations
+    .filter(d => d.type === 'declaration')
+    .reduce(
+      (obj, {property, value}) => Object.assign(obj, {[property]: value}),
+      {},
+    )
+  return {parsedRules}
+}
+
+function isSubset(styles, computedStyle) {
+  return Object.entries(styles).every(
+    ([prop, value]) => computedStyle.getPropertyValue(prop) === value,
+  )
+}
+
+function printoutStyles(styles) {
+  return Object.keys(styles)
+    .sort()
+    .map(prop => `${prop}: ${styles[prop]};`)
+    .join('\n')
+}
+
+// Highlights only style rules that were expected but were not found in the
+// received computed styles
+function expectedDiff(expected, computedStyles) {
+  const received = Array.from(computedStyles)
+    .filter(prop => expected[prop])
+    .reduce(
+      (obj, prop) =>
+        Object.assign(obj, {[prop]: computedStyles.getPropertyValue(prop)}),
+      {},
+    )
+  const diffOutput = jestDiff(
+    printoutStyles(expected),
+    printoutStyles(received),
+  )
+  // Remove the "+ Received" annotation because this is a one-way diff
+  return diffOutput.replace(`${chalk.red('+ Received')}\n`, '')
+}
+
+export function toHaveStyle(htmlElement, css) {
+  checkHtmlElement(htmlElement)
+  const {parsedRules: expected, parsingError} = parseCSS(css)
+  if (parsingError) {
+    return {
+      pass: this.isNot, // Fail regardless of the test being positive or negative
+      message: () => parsingError,
+    }
+  }
+  const received = getComputedStyle(htmlElement)
+  return {
+    pass: isSubset(expected, received),
+    message: () => {
+      const matcher = `${this.isNot ? '.not' : ''}.toHaveStyle`
+      return [
+        matcherHint(matcher, 'element', ''),
+        expectedDiff(expected, received),
+      ].join('\n\n')
+    },
+  }
+}

src/utils.js

@@ -1,3 +1,4 @@
+import redent from 'redent'
 import {
   RECEIVED_COLOR as receivedColor,
   EXPECTED_COLOR as expectedColor,
@@ -30,8 +31,8 @@ function getMessage(
 ) {
   return [
     `${matcher}\n`,
-    `${expectedLabel}:\n  ${expectedColor(expectedValue)}`,
-    `${receivedLabel}:\n  ${receivedColor(receivedValue)}`,
+    `${expectedLabel}:\n${expectedColor(redent(expectedValue, 2))}`,
+    `${receivedLabel}:\n${receivedColor(redent(receivedValue, 2))}`,
   ].join('\n')
 }