Add Turn Off Console Error Messages In A Test as a javascript til

Author: jbranchaud

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/).
 
-_677 TILs and counting..._
+_678 TILs and counting..._
 
 ---
 
@@ -268,6 +268,7 @@ _677 TILs and counting..._
 - [Timing Processes](javascript/timing-processes.md)
 - [Transforming ES6 and JSX With Babel 6](javascript/transforming-es6-and-jsx-with-babel-6.md)
 - [Truthiness of Integer Arrays](javascript/truthiness-of-integer-arrays.md)
+- [Turn Off Console Error Messages In A Test](javascript/turn-off-console-error-messages-in-a-test.md)
 - [Waiting On Multiple Promises](javascript/waiting-on-multiple-promises.md)
 - [Who Am I: NPM Edition](javascript/who-am-i-npm-edition.md)
 - [Yarn Commands Without The Emojis](javascript/yarn-commands-without-the-emojis.md)

javascript/turn-off-console-error-messages-in-a-test.md

@@ -0,0 +1,25 @@
+# Turn Off Console Error Messages In A Test
+
+I'm using [Jest](https://facebook.github.io/jest/) to test a React component
+that requires a prop via
+[PropTypes](https://reactjs.org/docs/typechecking-with-proptypes.html). In
+one of my tests, I want to test that component when the required prop is
+excluded. The side effect of doing this is that my test output gets
+cluttered with the PropType warning.
+
+The thing to do is silence the error message during that test.
+
+```javascript
+it('renders a component without a required prop', () => {
+  const originalError = console.error;
+  console.error = jest.fn();
+
+  // test code here
+  expect(shallow(<My Component />)).toDoSomething;
+
+  console.error = originalError;
+});
+```
+
+We can silence `console.error` by temporarily replacing it with a
+Jest-mocked function and then putting it back at the end of the test.