⚪ ️ 1.8 If needed, use only short & inline snapshots
:white_check_mark: Do: When there is a need for snapshot testing, use only short and focused snapshots (i.e. 3-7 lines) that are included as part of the test (Inline Snapshot) and not within external files. Keeping this guideline will ensure your tests remain self-explanatory and less fragile.
On the other hand, ‘classic snapshots’ tutorials and tools encourage storing big files (e.g. component rendering markup, API JSON result) over some external medium and ensure each time when the test runs to compare the received result with the saved version. This, for example, can implicitly couple our test to 1000 lines with 3000 data values that the test writer never read and reasoned about. Why is this wrong? By doing so, there are 1000 reasons for your test to fail - it’s enough for a single line to change for the snapshot to get invalid and this is likely to happen a lot. How frequently? for every space, comment, or minor CSS/HTML change. Not only this, the test name wouldn’t give a clue about the failure as it just checks that 1000 lines didn’t change, also it encourages the test writer to accept as the desired true a long document he couldn’t inspect and verify. All of these are symptoms of obscure and eager test that is not focused and aims to achieve too much
It’s worth noting that there are few cases where long & external snapshots are acceptable - when asserting on schema and not data (extracting out values and focusing on fields) or when the received document rarely changes
❌ Otherwise: A UI test fails. The code seems right, the screen renders perfect pixels, what happened? your snapshot testing just found a difference from the original document to the current received one - a single space character was added to the markdown…
✏ Code Examples
:thumbsdown: Anti-Pattern Example: Coupling our test to unseen 2000 lines of code
it("TestJavaScript.com is renderd correctly", () => {
//Arrange
//Act
const receivedPage = renderer
.create(<DisplayPage page="http://www.testjavascript.com"> Test JavaScript </DisplayPage>)
.toJSON();
//Assert
expect(receivedPage).toMatchSnapshot();
//We now implicitly maintain a 2000 lines long document
//every additional line break or comment - will break this test
});:clap: Doing It Right Example: Expectations are visible and focused
it("When visiting TestJavaScript.com home page, a menu is displayed", () => {
//Arrange
//Act
const receivedPage = renderer
.create(<DisplayPage page="http://www.testjavascript.com"> Test JavaScript </DisplayPage>)
.toJSON();
//Assert
const menu = receivedPage.content.menu;
expect(menu).toMatchInlineSnapshot(`
<ul>
<li>Home</li>
<li> About </li>
<li> Contact </li>
</ul>
`);
});