Add Tell Jest To Focus On Running Only One Test as a JavaScript TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1243 TILs and counting..._
+_1244 TILs and counting..._
 
 ---
 
@@ -451,6 +451,7 @@ _1243 TILs and counting..._
 - [Start Node Process In Specific Timezone](javascript/start-node-process-in-specific-timezone.md)
 - [String Interpolation With Template Literals](javascript/string-interpolation-with-template-literals.md)
 - [Support Nested Matching In Custom Jest Matchers](javascript/support-nested-matching-in-custom-jest-matchers.md)
+- [Tell Jest To Focus On Running Only One Test](javascript/tell-jest-to-focus-on-running-only-one-test.md)
 - [Tell Prettier To Not Format A Statement](javascript/tell-prettier-to-not-format-a-statement.md)
 - [Test Coverage Stats With Jest](javascript/test-coverage-stats-with-jest.md)
 - [Test Timing-Based Code With Jest Fake Timers](javascript/test-timing-based-code-with-jest-fake-timers.md)

javascript/tell-jest-to-focus-on-running-only-one-test.md

@@ -0,0 +1,34 @@
+# Tell Jest To Focus On Running Only One Test
+
+Test output can be noisy. Sometimes one test is inadvertently dependent on
+another. These are some of the reasons you may want to tell
+[Jest](https://jestjs.io/) to focus in and only run one particular `test`
+block.
+
+You can do this by calling
+[`test.only()`](https://jestjs.io/docs/setup-teardown#general-advice) instead
+of `test()`.
+
+Find the test block you are interested in focusing on and update it to look
+like this:
+
+```javascript
+// tests above ...
+
+test.only('ensure the function returns the value', () => {
+  // ...
+  // test implementation
+  // ...
+})
+
+// tests below ...
+```
+
+With that 5-character addition (`.only`) we instruct Jest to run that one test
+while skipping all others.
+
+This is particularly useful when you are doing some `console.log` debugging of
+a test. You can eliminate any confusion about which test is logging out by only
+running one test.
+
+[source](https://stackoverflow.com/a/42828586/535590)