Add Configure Jest To Run A Test Setup File as a javascript til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_670 TILs and counting..._
+_671 TILs and counting..._
 
 ---
 
@@ -229,6 +229,7 @@ _670 TILs and counting..._
 - [Character Codes from Keyboard Listeners](javascript/character-codes-from-keyboard-listeners.md)
 - [Check If Something Is An Array](javascript/check-if-something-is-an-array.md)
 - [Computed Property Names In ES6](javascript/computed-property-names-in-es6.md)
+- [Configure Jest To Run A Test Setup File](javascript/configure-jest-to-run-a-test-setup-file.md)
 - [Create An Array Containing 1 To N](javascript/create-an-array-containing-1-to-n.md)
 - [Create Bootstrapped Apps With Yarn](javascript/create-bootstrapped-apps-with-yarn.md)
 - [Custom Type Checking Error Messages With Yup](javascript/custom-type-checking-error-messages-with-yup.md)

javascript/configure-jest-to-run-a-test-setup-file.md

@@ -0,0 +1,27 @@
+# Configure Jest To Run A Test Setup File
+
+Jest can be configured to run a setup file before each test. This is useful
+for configuring your testing framework in a single place, rather than in
+each test file.
+
+This setup file can be specified in `package.json` (or `jest.config.js`).
+
+```javascript
+// package.json
+{
+  // ...
+  "jest": {
+    "setupTestFrameworkScriptFile": "<rootDir>src/setupTests.js"
+  }
+}
+```
+
+The
+[`setupTestFrameworkScriptFile`](https://facebook.github.io/jest/docs/en/configuration.html#setuptestframeworkscriptfile-string)
+points to a test setup file at the specified location rooted at `<rootDir>`
+(the root of your project).
+
+This kind of setup is helpful for something like
+[Enzyme](https://github.com/airbnb/enzyme/blob/master/docs/guides/jest.md)
+that needs to be configured with a specific adapter for use throughout your
+tests.