Add snapshot testing FAQs

docs/SnapshotTesting.md

@@ -55,7 +55,7 @@ One such situation can arise if we intentionally change the address the Link com
 
 ```javascript
 // Updated test case with a Link to a different address
-it('renders correctly', () => {  
+it('renders correctly', () => {
   const tree = renderer.create(
     <Link page="http://www.instagram.com">Instagram</Link>
   ).toJSON();
@@ -93,11 +93,49 @@ Date.now = jest.fn(() => 1482363367071);
 
 Now, every time the snapshot test case runs, `Date.now()` will return `1482363367071` consistently. This will result in the same snapshot being generated for this component regardless of when the test is run.
 
-## React, React Native and Snapshot Testing
+## Frequently Asked Questions
+
+### Should the snapshot files be committed?
+
+Yes, the snapshot files should be committed alongside the modules they are covering.
+In fact, snapshots represent the state of the source modules at any given point in time.
+In this way, when the source modules are modified, Jest can tell what changed from the previous version.
+
+### Does snapshot testing work with React components only?
+
+One of best use cases for snapshot testing is [React](/jest/docs/tutorial-react.html) and [React Native](/jest/docs/tutorial-react-native.html) components.
+When engineers work with components, they want to make sure the rendered output doesn't change unexpectedly, and that's what snapshot testing is for.
+However, snapshots can capture any serializable value and should be used anytime the goal is testing whether the output is correct.
+Typical examples are [snapshotting CLI output](https://github.com/facebook/jest/blob/master/integration_tests/__tests__/console-test.js) or API responses.
+
+### Does code coverage work with snapshots testing?
+
+Yes, it works very well.
+
+### What's the difference between snapshot testing and visual regression testing?
+
+Snapshot testing and visual regression testing are two distinct ways of testing UIs, and they serve different purposes.
+Visual regression testing tools take screenshots of web pages and compare the resulting images pixel by pixel.
+With Snapshot testing values are serialized, stored within text files and compared using a diff algorithm.
+
+### Does snapshot testing substitute unit testing?
+
+The aim of snapshot testing is not replacing existing unit tests, but providing additional value and making testing painless.
+In some scenarios, snapshot testing can potentially remove the need for unit testing for a particular set of functionalities (e.g. React components), but they can work together as well.
+
+### What is the performance of snapshot testing regarding speed and size of the generated files?
+
+Jest has been rewritten with performance in mind, and snapshot testing is not an exception. Since snapshots are stored within text files, this way of testing is fast and reliable.
+You should be aware that Jest generates a new file for each test invoking the `toMatchSnapshot` matcher.
+If used correctly, the size of the snapshots is pretty small, and it doesn't negatively affect your codebase.
+For reference, the size of the snapshot files in the entire Jest codebase is less than 300 KB.
+
+### How do I resolve conflicts within snapshot files?
 
-As you've learned, snapshot testing was built to make it easier to write tests for React and React Native. Check out the [React tutorial](/jest/docs/tutorial-react.html) and the [React Native tutorial](/jest/docs/tutorial-react-native.html) to get started with snapshot testing on your React or React Native application.
+Snapshot files must always represent the current state of the modules they are covering.
+Therefore, if you are merging two branches and encounter a conflict in the snapshot files, the best solution is updating the snapshots to the latest version.
 
-## Additional Uses
+### Is it possible to apply test-driven development principles with snapshot testing?
 
-Snapshots can capture any serializable value. Common examples are [snapshotting CLI output](https://github.com/facebook/jest/blob/master/integration_tests/__tests__/console-test.js
-) or API responses.
+Although it would be possible to generate snapshot files manually, that's not the purpose of snapshot testing.
+Snapshots help figuring out whether the output of the modules covered by tests is changed, rather than giving some guidance to design the code.