Add Mock A Function With Return Values Using Jest 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/).
 
-_657 TILs and counting..._
+_658 TILs and counting..._
 
 ---
 
@@ -246,6 +246,7 @@ _657 TILs and counting..._
 - [Link A JavaScript Package Locally](javascript/link-a-javascript-package-locally.md)
 - [List Top-Level NPM Dependencies](javascript/list-top-level-npm-dependencies.md)
 - [Matching Multiple Values In A Switch Statement](javascript/matching-multiple-values-in-a-switch-statement.md)
+- [Mock A Function With Return Values Using Jest](javascript/mock-a-function-with-return-values-using-jest.md)
 - [New Dates Can Take Out Of Bounds Values](javascript/new-dates-can-take-out-of-bounds-values.md)
 - [Numbers Are Empty](javascript/numbers-are-empty.md)
 - [Object Initialization With Shorthand Property Names](javascript/object-initialization-with-shorthand-property-names.md)

javascript/mock-a-function-with-return-values-using-jest.md

@@ -0,0 +1,39 @@
+# Mock A Function With Return Values Using Jest
+
+[Jest](https://facebook.github.io/jest/) provides a collection of utilities
+for working with mocked functions.  To create a mock function, do:
+
+```javascript
+jest.fn()
+
+// assign it to a variable
+const fakeFunc = jest.fn();
+
+// pass it as a prop
+<SpecialInput handleChange={jest.fn()} />
+```
+
+A mocked function can then be attributed with a return value.
+
+```javascript
+const fakeFunc = jest.fn();
+fakeFunc.mockReturnValue("hello");
+fakeFunc(); // => "hello"
+```
+
+The
+[`mockReturnValue()`](https://facebook.github.io/jest/docs/en/mock-function-api.html#mockfnmockreturnvaluevalue)
+function ensures that the value is returned whenever your function is
+called.
+
+We can also limit the return value to occurring just once.
+
+```javascript
+const fakeFunc = jest.fn();
+fakeFunc.mockReturnValueOnce("hello");
+fakeFunc(); // => "hello"
+fakeFunc(); // => null
+```
+
+[`mockReturnValueOnce()`](https://facebook.github.io/jest/docs/en/mock-function-api.html#mockfnmockreturnvalueoncevalue)
+ensures the value is returned once and all subsequent calls yield `null`.