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

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

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+# 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.