Testing Stateful React Function Components with React Testing Library

Testing Stateful React Function Components with React Testing Library

With the introduction of React Hooks, there is more of an incentive to write function components in React since there is no longer a need use classes when building components.

With the introduction of React Hooks, there is more of an incentive to write function components in React since there is no longer a need use classes when building components.

When it comes to testing React components, there are two popular libraries that are often reached for: enzyme and react-testing-library. I used to always reach for enzyme when testing my react components, but I've recently made the switch to react-testing-library because the react-testing-library API encourages tests that are completely ignorant of implementation details. Why is this good? Testing implementation details can lead to both false negatives and false positives, which make tests much more unreliable.

In this post, I'll look at an example stateful function component that is tested with react-testing-library. I'll also write the same component into its class component equivalent and show how the class component can be tested with enzyme.

Checklist Example

Here's a checklist component that allows a user to check off items and display a message after all the items have been checked.

Note: All these examples are written in TypeScript.

export const Checklist = ({ items }: ChecklistProps) => {
    const [checklistItems, setChecklistItems] = useState(items);

    const handleClick = (itemIndex: number) => {
        const toggledItem = { ...checklistItems[itemIndex] };
        toggledItem.completed = !toggledItem.completed;
        setChecklistItems([...checklistItems.slice(0, itemIndex), toggledItem, ...checklistItems.slice(itemIndex + 1)]);
    };

    // Determine if all tasks are completed
    const allTasksCompleted = checklistItems.every(({ completed }) => completed);

    return (
        <div>
            <form>
                {checklistItems.map((item, index) => (
                    <React.Fragment key={item.description}>
                        <input
                            onChange={() => handleClick(index)}
                            type="checkbox"
                            className="checkbox"
                            checked={item.completed ? true : false}
                            id={item.description}
                        />
                        <label htmlFor={item.description}>{item.description}</label>
                    </React.Fragment>
                ))}
            </form>
            <TasksCompletedMessage className="xs-text-4 text-green xs-mt2" visible={allTasksCompleted}>
                All tasks completed{' '}
                <span role="img" aria-label="checkmark">
                    ✅
                </span>
            </TasksCompletedMessage>
        </div>
    );
};

Here's what the component would look like when used:

Checklist with two unchecked items.

Checklist with two checked items and a message indicating all items are checked.

Now when I'm thinking of testing this component, I want to make sure that a user is able to properly select a checkbox and also display the completed message when all the items have been checked. Here's how these tests would look like when written with react-testing-library:

afterEach(cleanup);

const mockItems = [
    {
        description: 'first item',
        completed: false,
    },
    {
        description: 'second item',
        completed: false,
    },
    {
        description: 'third item',
        completed: false,
    },
];

describe('Checklist', () => {
    it('should check two out the three checklist items', () => {
        const { getByText, getByLabelText } = render(<Checklist items={mockItems} />);

        fireEvent.click(getByText('first item'));
        fireEvent.click(getByText('second item'));

        expect(getByLabelText('first item').checked).toBe(true);
        expect(getByLabelText('second item').checked).toBe(true);
        expect(getByLabelText('third item').checked).toBe(false);
        expect(getByText('All tasks completed')).not.toBeVisible();
    });

    it('should display a message when all items are completed', () => {
        const { getByText, getByLabelText } = render(<Checklist items={mockItems} />);

        fireEvent.click(getByText('first item'));
        fireEvent.click(getByText('second item'));
        fireEvent.click(getByText('third item'));

        expect(getByLabelText('first item').checked).toBe(true);
        expect(getByLabelText('second item').checked).toBe(true);
        expect(getByLabelText('third item').checked).toBe(true);
        expect(getByText('All tasks completed')).toBeVisible();
    });
});

There are a few special things of note in these tests. The first being how we are targeting elements on the page by their text rather than by a class name, id, or other DOM selector. This is important because this is actually how a user will find an element on the page. A user doesn't see or care about what classes or ids are found on an element so it's unrealistic to expect a user to find and interact with an element on a page based on a DOM selector.

react-testing-library doesn't only allow you to target elements by text, but you can also target elements through labels, placeholder text, alt text, title, display value, role, and test id (see the documentation for details on each of these methods of targeting elements).

Another important thing to notice in these tests is that we aren't looking at the value for the internal component state, nor are we testing any of the functions being used within the component itself. Basically what this means is that we don't care about testing the implementation details of our component, but we are more interested in testing how the component will actually be used by a user. Actually, it's extremely difficult to test implementation details of a function component since it's not possible to access the component state, nor can we access any of the functions/methods that are defined and used inside of the component. However, as a fun exercise, let's look at our checklist component written in as a class component:

export class Checklist extends React.Component<ChecklistProps, ChecklistState> {
    state = {
        checklistItems: this.props.items,
    };

    handleChange = (itemIndex: number) => {
        const toggledItem = { ...this.state.checklistItems[itemIndex] };
        toggledItem.completed = !toggledItem.completed;
        this.setState({
            checklistItems: [
                ...this.state.checklistItems.slice(0, itemIndex),
                toggledItem,
                ...this.state.checklistItems.slice(itemIndex + 1),
            ],
        });
    };

    render() {
        // Determine if all tasks are completed
        const allTasksCompleted = this.state.checklistItems.every(({ completed }) => completed);
        return (
            <div>
                <form>
                    {this.state.checklistItems.map((item, index) => (
                        <React.Fragment key={item.description}>
                            <input
                                onChange={() => this.handleChange(index)}
                                type="checkbox"
                                className="checkbox"
                                checked={item.completed ? true : false}
                                id={item.description}
                            />
                            <label htmlFor={item.description}>{item.description}</label>
                        </React.Fragment>
                    ))}
                </form>
                <TasksCompletedMessage className="xs-text-4 text-green xs-mt2" visible={allTasksCompleted}>
                    All tasks completed{' '}
                    <span role="img" aria-label="checkmark">
                        ✅
                    </span>
                </TasksCompletedMessage>
            </div>
        );
    }
}

Now, let's use enzyme to test our checklist class component. However, this time we will be testing the implementation details of our component.

const mockItems = [
    {
        description: 'first item',
        completed: false,
    },
    {
        description: 'second item',
        completed: false,
    },
    {
        description: 'third item',
        completed: false,
    },
];

describe('Checklist Class Component', () => {
    it('should render all 3 list items', () => {
        const wrapper = mount(<Checklist items={mockItems} />);

        expect(wrapper.find('label').length).toBe(3);
    });

    describe('handleChange', () => {
        it('should check two out the three checklist items', () => {
            const wrapper = mount(<Checklist items={mockItems} />);
            const instance = wrapper.instance();

            instance.handleChange(0);
            instance.handleChange(1);

            expect(wrapper.state('checklistItems')).toEqual([
                {
                    description: 'first item',
                    completed: true,
                },
                {
                    description: 'second item',
                    completed: true,
                },
                {
                    description: 'third item',
                    completed: false,
                },
            ]);
        });

        it('should display a message when all items are completed', () => {
            const wrapper = mount(<Checklist items={mockItems} />);
            const instance = wrapper.instance();

            instance.handleChange(0);
            instance.handleChange(1);
            instance.handleChange(2);
            wrapper.update();

            expect(
                wrapper
                    .find('.text-green')
                    .first()
                    .props().visible,
            ).toBe(true);
        });
    });
});

Because the enzyme API makes available a component's state as well as the class methods of component (by accessing the component's instance), we are now able to test both those things. For example, looking at the test labelled should check two out the three checklist items, the handleChange method is triggered twice (which should happen when a user clicks two checklist items) and then the value of the state is checked to make sure it has updated appropriately. The problem with this test is that we aren't testing how this component is actually being used. The user doesn't care about the value of a component's internal state or if a function has been called. All a user cares about (in this case) is that they are able to click on two checklist items and that both those checklist items appear as checked to them.

Enzyme's API doesn't allow for an element to be find by it's text, it only allows for elements to be selected based on a CSS selector, React component constructor, React component display name, or based on a component's props (see here for details on Enzyme selectors). Because Enzyme's API basically pushes you to test implementation details for a component, I prefer to stay away from Enzyme and instead use react-testing-library.

Refactoring Class Components to Function Components

Another advantage of using react-testing-library and not testing for implementation details is that you can easily refactor your class component to a function component without having the also refactor your tests. Think about it, if you're targeting class methods in your tests, those methods will no longer be available when it's being implemented within a function component.

Demo Repository

I've setup a demo repository, that contains the above example with the checklist and I've also created another example for a component named SelectTwo, which is a list of items that only allows for 2 items to be selected at once.

View the repo

Other Ressources

Here are some great ressources that you should check out if you're interested in learning more about react-testing-library.

What is JavaScript – All You Need To Know About JavaScript

What is JavaScript – All You Need To Know About JavaScript

In this article on what is JavaScript, we will learn the basic concepts of JavaScript.

After decades of improvement, JavaScript has become one of the most popular programming languages of all time. It all started in the year 1995 when Brendan Eich created JavaScript in a span of 10 days. Since then, it has seen multiple versions, updates and has grown to the next level.

Here’s a list of topics that I’ll be covering in this blog:

  1. What is JavaScript
  2. What can JavaScript do?
  3. JavaScript Frameworks
  4. The Big Picture: HTML, CSS & JavaScript
  5. Benefits of JavaScript
  6. Fundamentals of JavaScript
    VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case
What is JavaScript?

JavaScript is a high level, interpreted, programming language used to make web pages more interactive.

Have you ever thought that your website is missing something? Maybe it’s not engaging enough or it’s not as creative as you want it to be. JavaScript is that missing piece which can be used to enhance web pages, applications, etc to provide a more user-friendly experience.

What is JavaScript?

JavaScript is the language of the web, it is used to make the web look alive by adding motion to it. To be more precise, it’s a programming language that let’s you implement complex and beautiful things/design on web pages. When you notice a web page doing more than just sit there and gawk at you, you can bet that the web page is using JavaScript.

Feature of JavaScript

Scripting language and not Java: In fact, JavaScript has nothing to do with Java. Then why is it called “Java” Script? When JavaScript was first released it was called Mocha, it was later renamed to LiveScript and then to JavaScript when Netscape (founded JavaScript) and Sun did a license agreement. Object-based scripting language which supports polymorphism, encapsulation and to some extent inheritance as well.**Interpreted language: **It doesn’t have to be compiled like Java and C which require a compiler.JavaScript runs in a browser: You can run it on Google Chrome, Internet Explorer, Safari, etc. JavaScript can execute not only in the browser but also on the server and any device which has a JavaScript Engine.

What is JavaScript – Stackoverflow stats

Currently, we have 100s of programming languages and every day new languages are being created. Among these are few powerful languages that bring about big changes in the market and JavaScript is one of them.

JavaScript has always been on the list of popular programming languages. According to StackOverflow, for the 6th year in a row, JavaScript has remained the most popular and commonly used programming language.

What can JavaScript do?

JavaScript is mainly known for creating beautiful web pages & applications. An example of this is Google Maps. If you want to explore a specific map, all you have to do is click and drag with the mouse. And what sort of language could do that? You guessed it! It’s JavaScript.JavaScript can also be used in smart watches. An example of this is the popular smartwatch maker called Pebble. Pebble has created Pebble.js which is a small JavaScript Framework that allows a developer to create an application for the Pebble line of watches in JavaScript.

What is JavaScript – Applications of JavaScript
Most popular websites like Google, Facebook, Netflix, Amazon, etc make use of JavaScript to build their websites.Among things like mobile applications, digital art, web servers and server applications, JavaScript is also used to make Games. A lot of developers are building small-scale games and apps using JavaScript.## JavaScript Frameworks

One major reason for the popularity of JavaScript is the JavaScript Frameworks. Here’s a brief introduction of the most trending JavaScript frameworks :

  1. AngularJS is Google’s web development framework which provides a set of modern development and design features for rapid application development.

  2. ReactJS is another top JavaScript framework mainly maintained by Facebook and it’s behind the User Interface of Facebook and Instagram, showing off its efficiency in maintaining such high traffic applications.

What is JavaScript – JavaScript Frameworks

  1. MeteorJS is mainly used for providing back-end development. Using JavaScript on the back-end to save time and build expertise is one of the major ideas behind Meteor.

  2. jQuery can be used when you want to extend your website and make it more interactive. Companies like Google, WordPress and IBM rely on jQuery.

The Big Picture: HTML, CSS & JavaScript

Anyone familiar with JavaScript knows that it has something to do with HTML and CSS. But what is the relationship between these three? Let me explain this with an analogy.

What is JavaScript – HTML, CSS and JavaScript

Think of HTML (HyperText Markup Language) as the skeleton of the web. It is used for displaying the web.

On the other hand, CSS is like our clothes. We put on fashionable clothes to look better. Similarly, the web is quite stylish as well. It uses CSS which stands for Cascading Style Sheets for styling purpose.

Then there is JavaScript which puts life into a web page. Just like how kids move around using the skateboard, the web also motions with the help of JavaScript.

Benefits of JavaScript

There has to be a reason why so many developers love working on JavaScript. Well, there are several benefits of using JavaScript for developing web applications, here’s a few benefits:

It’s easy to learn and simple to implement. It is a weak-type programming language unlike the strong-type programming languages like Java and C++, which have strict rules for coding.

It’s all about being fast in today’s world and since JavaScript is mainly a client-side programming language, it is very fast because any code can run immediately instead of having to contact the server and wait for an answer.

Rich set of frameworks like AngularJS, ReactJS are used to build web applications and perform different tasks.

**Builds interactive websites: **We all get attracted to beautifully designed websites and JavaScript is the reason behind such attractive websites and applications.

JavaScript is an interpreted language that does not require a compiler because the web interprets JavaScript. All you need is a browser like Google Chrome or Internet Explorer and you can do all sorts of stuff in the browser.

JavaScript is platform independent and it is supported by all major browsers like Internet Explorer, Google Chrome, Mozilla Firefox, Safari, etc.

JavaScript Fundamentals

In this What is JavaScript blog, we’ll cover the following basic fundamentals of JavaScript
VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case## Variables

Variable is a name given to a memory location which acts as a container for storing data temporarily. They are nothing but reserved memory locations to store values.

What is JavaScript – Variables

To declare a variable in JavaScript use the ‘let’ keyword. For example:

let age;
age=22;

In the above example, I’ve declared a variable ‘age’ by using the ‘let’ keyword and then I’ve stored a value (22) in it. So here a memory location is assigned to the ‘age’ variable and it contains a value i.e. ’22’.

Constants

Constants are fixed values that don’t change during execution time.

To declare a constant in JavaScript use the ‘const’ keyword. For example:

const mybirthday;
mybirthday='3rd August'; 

Data types

You can assign different types of values to a variable such as a number or a string. In JavaScript, there are two categories of data types :

What is JavaScript – Data Types

Objects

An object is a standalone entity with properties and types and it is a lot like an object in real life. For example, consider a girl, whose name is Emily, age is 22 and eye-color is brown. In this example the object is the girl and her name, age and eye-color are her properties.

What is JavaScript – Objects example

Objects are variables too, but they contain many values, so instead of declaring different variables for each property, you can declare an object which stores all these properties.

To declare an object in JavaScript use the ‘let’ keyword and make sure to use curly brackets in such a way that all property-value pairs are defined within the curly brackets. For example:

let girl= {
name: 'Emily',
age: 22,
eyeColour: 'Brown'
};

In the above example, I’ve declared an object called ‘girl’ and it has 3 properties (name, age, eye colour) with values (Emily, 22, Brown).

Arrays

An array is a data structure that contains a list of elements which store multiple values in a single variable.

For example, let’s consider a scenario where you went shopping to buy art supplies. The list of items you bought can be put into an array.

What is JavaScript – Arrays example

To declare an array in JavaScript use the ‘let’ keyword with square brackets and all the array elements must be enclosed within them. For example:

let shopping=[];
shopping=['paintBrush','sprayPaint','waterColours','canvas'];

In the above example I’ve declared an array called ‘shopping’ and I’ve added four elements in it.

Also, array elements are numbered from zero. For example this is how you access the first array element:

shopping[0];		

Functions

A function is a block of organised, reusable code that is used to perform single, related action.

Let’s create a function that calculates the product of two numbers.

To declare a function in JavaScript use the ‘function’ keyword. For example:

function product(a, b) {
return a*b;
}

In the above example, I’ve declared a function called ‘product’ and I’ve passed 2 parameters to this function, ‘a’ and ‘b’ which are variables whose product is returned by this function. Now, in order to call a function and pass a value to these parameters you’ll have to follow the below syntax:

product(8,2);

In the above code snippet I’m calling the product function with a set of values (8 & 2). These are values of the variables ‘a’ and ‘b’ and they’re called as arguments to the function.

Conditional statements – if

Conditional statement is a set of rules performed if a certain condition is met. The ‘if’ statement is used to execute a block of code, only if the condition specified holds true.

What is JavaScript – if flowchart

To declare an if statement in JavaScript use the ‘if’ keyword. The syntax is:

if(condition) {
statement;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[2]) {
console.log('Correct!');
}

In the above example I’ve defined an array of numbers and then I’ve defined an if block. Within this block is a condition and a statement. The condition is ‘(numbers[0]==numbers[2])’ and the statement is ‘console.log(‘Correct!’)’. If the condition is met, only then the statement will be executed.

Conditional statements- Else if

Else statement is used to execute a block of code if the same condition is false.

What is JavaScript – Else-if flowchart

The syntax is:

if(condition) {
statement a;
}
else (condition) {
statement b;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[4] {
console.log("Correct!");
}
else {
console.log("Wrong, please try again");
}

In the above example, I’ve defined an if block as well as an else block. So if the conditions within the if block holds false then the else block gets executed. Try this for yourself and see what you get!

**Loops **

Loops are used to repeat a specific block until some end condition is met. There are three categories of loops in JavaScript :

  1. while loop
  2. do while loop
  3. for loop
While loop

While the condition is true, the code within the loop is executed.

What is JavaScript – while loop flowchart

The syntax is:

while(condition) {
loop code;
}

Now let’s look at an example:

let i=0;
while(i < 5) {
console.log("The number is " +i);
i++;
}

In the above example, I’ve defined a while loop wherein I’ve set a condition. As long as the condition holds true, the while loop is executed. Try this for yourself and see what you get!

Do while loop

This loop will first execute the code, then check the condition and while the condition holds true, execute repeatedly.

What is JavaScript – Do while loop flowchart

Refer the syntax to better understand it:

do {
loop code;
} while(condition);

This loop executes the code block once before checking if the condition is true, then it will repeat the loop as long as the condition holds true.

Now let’s look at an example:

do {
console.log("The number is " +i);
i++;
}
while(i > 5);

The above code is similar to the while loop code except, the code block within the do loop is first executed and only then the condition within the while loop is checked. If the condition holds true then the do loop is executed again.

For loop

The for loop repeatedly executes the loop code while a given condition is TRUE. It tests the condition before executing the loop body.

What is JavaScript – for loop flowchart

The syntax is:

for(begin; condition; step) {
loop code;
}

In the above syntax:

  • begin statement is executed one time before the execution of the loop code
  • condition defines the condition for executing the loop code
  • step statement is executed every time after the code block has been executed

For example:

for (i=0;i<5;i++) {
console.log("The number is " +i);
}

In the above example, I’ve defined a for loop within which I’ve defined the begin, condition and step statements. The begin statement is that ‘i=0’. After executing the begin statement the code within the for loop is executed one time. Next, the condition is checked, if ‘i<5’ then, the code within the loop is executed. After this, the last step statement (i++) is executed. Try this and see what you get!

Switch Case

The switch statement is used to perform different actions based on different conditions.

What is JavaScript – Switch case flowchart

Let’s look at the syntax for switch case:

switch(expression) {
case 1:
code block 1
break;
case 2:
code block 2
break;
default:
code block 3
break;
}

How does it work?

  • Switch expression gets evaluated once
  • Value of the expression is compared with the values of each case
  • If there is a match, the associated block of code is executed

Let’s try this with an example:

let games='football';
switch(games) {
case "throwball":
console.log("I dislike throwball!");
break;
case "football":
console.log("I love football!");
break;
case "cricket":
console.log("I'm a huge cricket fan!");
break;
default:
console.log("I like other games");
break;
}

In the above example the switch expression is ‘games’ and the value of games is ‘football’. The value of ‘games’ is compared with the value of each case. In this example it is compared to ‘throwball’, ‘cricket’ and ‘football’. The value of ‘games’ matches with the case ‘football’, therefore the code within the ‘football’ case is executed. Try this for yourself and see what you get!

With this, we come to the end of this blog. I hope you found this blog informative and I hope you have a basic understanding of JavaScript. In my next blog on JavaScript I’ll be covering in-depth concepts, so stay tuned.

Also, check out our video on JavaScript Fundamentals if you want to get started as soon as possible and don’t forget to leave a comment if you have any doubt and also, let us know whether you’d want us to create more content on JavaScript. We are listening!