Add Mock A Function With Return Values Using Jest as a javascript til

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

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+# 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`.