Rich Events and Testing

Add event handling to a stateful class component by first writing tests during development.

Our counter component has local state, but doesn't respond to clicks. We need to add event handlers, but we'd like to do the same "fail faster" approach:

  • Write tests first
  • Use type information to give us red squigglies when we mess up
  • Don't go to the browser until the end

As in previous steps, start with the following setup:

  • Counter.tsx and Counter.test.tsx in side-by-side tabs

  • Start the Jest run configuration

  • Stop the start run configuration


The finished code for this tutorial step is in the repository.

First Failing Test

Let's start with a failing test that clicks on the count and checks if the number is updated. In Counter.test.tsx, clone the first test and change it as follows:

import { render, fireEvent } from "@testing-library/react";
// ...

test("should increment the count by one", () => {
	const { getByTitle } = render(<Counter />);
	const counter = getByTitle("Current Count");

This test renders the Counter component, checks its initial value, pretends to click, then checks the final value.

fireEvent, what's that? It's the big idea in this tutorial step. You can pretend to click, or dispatch other DOM events, even without a real browser or "mouse". Jest uses the browser-like JSDOM environment entirely inside NodeJS to fire the event.

This new test fails: the number didn't increment. Which is good!

What's really good: that was dead simple. In one line we automated doing a click and checking the result. Much more pleasurable than switching to the browser and clicking everything in your app, on every change, to see if your change works without breaking the universe.

onClick Handler

The component doesn't handle clicks. Let's head to Counter.tsx and add a click handler on the <span>:

	title="Current Count"
	onClick={() => this.setState({ count: this.state.count + 1 })}

Our test passes. But that's putting some logic into the component TSX. Also, inline arrow functions inside the render can have a performance impact.

Let's move it out into a class method, which also makes that behavior easier to test in isolation:

incrementCounter = () => {
  this.setState({ count: this.state.count + 1 });

render() {
  const { label = "Count" } = this.props;
  return (
      <span title="Count Label">{label}</span>
        title="Current Count"

The above was a bit of a lie: this isn't strictly a class method. It is a property that is an arrow function. This solves the issue of binding, allowing this to be bound to the component instance, not the event.

The incrementCounter arrow function actually gets an event passed in, which we aren't using. Let's add it in:

incrementCounter = (event) => {
	this.setState({ count: this.state.count + 1 });

This works but TypeScript gives a compiler error. Our tsconfig.json disallows implicit any:

Error:(21, 23) TS7006: Parameter 'event' implicitly has an 'any' type.

That's easy enough to solve:

incrementCounter = (event: any) => {
	this.setState({ count: this.state.count + 1 });

But that's cheating. What type is that event? It's a MouseEvent. Let's put the correct typing on the argument:

incrementCounter = (event: React.MouseEvent<HTMLElement>) => {
	this.setState({ count: this.state.count + 1 });

Ugh, that's a lot of keystrokes. Is it worth it? Let's show why. First, inincrementCounter, let's determine the value to increment by, first as a mistake:

incrementCounter = (event: React.MouseEvent<HTMLElement>) => {
	const inc = 10 ? event.shiftKey : 1;
	this.setState({ count: this.state.count + inc });

TypeScript told us that we were trying to add a boolean to a number. Let's set the type of inc explicitly, instead of inferring it. Our first fix:

const inc: number = 10 ? event.shiftKey : 1;

That's closer. TypeScript now moves the error to the correct line:

Error:(28, 31) TS2365: Operator '+' cannot be applied to types
'number' and 'boolean | 1'.

We see that we have the order wrong on the ternary...a frequent, maddening, easy-to-miss error: Here's the correct version:

const inc: number = event.shiftKey ? 10 : 1;

Our test now passes.

Advance By Ten with Shift-Click

Let's add one more feature: if you click with the Shift key pressed, you increase the count by 10.

create-react-library includes Testing Library which, as we saw above, bundles simple event firing. We need to simulate "modifiers" to an event: a shift pressed during a mouse event. For simulating more complex events, we need to install user-event:

npm install @testing-library/user-event @testing-library/dom --save-dev

Note: After installing this, you might get a TypeScript error in pretty-format/build/index.d.ts. If so, go to your package.json and upgrade the typescript version to be ^3.8.3 then npm install.

Once installed, we import userEvent at the top of Counter.test.tsx:

import userEvent from "@testing-library/user-event";

On to the feature, first by writing a test. Clone the last test and change it to the following:

test("should increment the count by ten", () => {
	const { getByTitle } = render(<Counter />);
	const counter = getByTitle("Current Count");
	expect(counter).toHaveTextContent("0");, { shiftKey: true });

In this test, we changed from fireEvent in testing-library to userEvent in user-event. The click passes in some information saying shiftKey was "pressed".

The test passes! Our code in Counter.handleIncrement above worked and is now tested. Even better, we did this entire step -- adding a multi-featured click incrementer -- without visiting the browser.

Which we'll do now. As we have been doing at the end, give this a try in the browser by firing up the start run configuration and clicking, then shift-clicking, on the counter. When done, terminate the start process.