Add create-react-app Has A Default Test Setup File as a react 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/).
 
-_671 TILs and counting..._
+_672 TILs and counting..._
 
 ---
 
@@ -459,6 +459,7 @@ _671 TILs and counting..._
 - [Building A React App In The Browser](react/building-a-react-app-in-the-browser.md)
 - [Create Dynamically Named Custom React Components](react/create-dynamically-named-custom-react-components.md)
 - [create-react-app Comes With Lodash](react/create-react-app-comes-with-lodash.md)
+- [create-react-app Has A Default Test Setup File](react/create-react-app-has-a-default-test-setup-file.md)
 - [Debug Jest Tests In create-react-app](react/debug-jest-tests-in-create-react-app.md)
 - [Defining State In A Simple Class Component](react/defining-state-in-a-simple-class-component.md)
 - [Destructure Variables As Props To A Component](react/destructure-variables-as-props-to-a-component.md)

react/create-react-app-has-a-default-test-setup-file.md

@@ -0,0 +1,20 @@
+# create-react-app Has A Default Test Setup File
+
+In [_Configure Jest To Run A Test Setup
+File_](https://github.com/jbranchaud/til/blob/master/javascript/configure-jest-to-run-a-test-setup-file.md),
+I pointed to a way of configuring Jest in either the `package.json` or
+`jest.config.js` file with the `setupTestFrameworkScriptFile` value.
+
+In a `create-react-app` project, this is not an option because
+`setupTestFrameworkScriptFile` is not one of the permitted config values for
+Jest.
+
+There is a built-in value which happens to match what was recommended in the
+above post -- `<rootDir>src/setupTests.js`.
+
+This means that there is no configuration required. Instead, just create a
+`setupTests.js` file in the `src` directory of your CRA project and add any
+framework setup you need there. That file is already configured to run when
+you invoke `yarn test`.
+
+[source](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#initializing-test-environment)