Facebook’s React provides developers with a lightweight frontend framework that creates views using a blend of Javascript and HTML in what are known as components. While React can be written using pure Javascript, our team constructs these components using JSX - our team prefers it because it dramatically improves readability and has no impact on performance since it transcompiles to pure Javascript on every build. While it will take time to transcompile, a modern web app will need a build step anyway.
Components are designed to be modular and reusable, which actually makes them great for unit testing! This post assumes you have a moderate understanding of React, but let’s look at an instance of two React classes that interact with each other - a simple TodoList
and some Todo
s.
import React from "react";
import Todo from "components/todo";
class TodoList extends React.Component{
render(){
var { todoTexts } = this.props;
return (
<div className="todo-list">
<span className="greeting">Welcome. Here are your todos.</span>
{
todoTexts.map( (todo, index) => {
return (<Todo key={ `todo-${index}` } text={ todo } />);
})
}
</div>
);
}
}
TodoList.displayName = "TodoList";
export default TodoList;
This TodoList
is pretty straightforward - it greets the user with a simple span
and then lists as many Todo
s as were given to it via its parent (let’s assume the parent of the Todo
is contacting a store or API or whatever to gather exactly what are the todos).
Now, like good developers, let’s establish a test suite to ensure that our TodoList
does exactly what we want it to.
Enter: TestUtils!
React provides a nice set of test utilities that allow us to inspect and examine the components we build.
There are several different uses, but I want to discuss the use of Shallow Rendering. Shallow rendering allows us to inspect the results of a component’s render
function, and see check what HTML and React components that particular component returns.
So, just to be sure, let’s make sure that the TodoList
returns an element with class “todo-list”, that it has a child element that greets the user (we can guess this by it having a class “greeting”), and that it will render Todo
s for all of the Todos that are given to it.
A test suite for that might look like this:
import chai from "chai";
import jsxChai from "jsx-chai";
import React from "react";
import ReactDOM from "react-dom";
import TestUtils from "react-addons-test-utils";
import shallowTestUtils from "react-shallow-testutils";
import TodoList from "app/components/todolist";
import Todo from "app/components/todo";
chai.use(jsxChai);
var expect = chai.expect;
describe("TodoList", function(){
var renderer, todolist;
beforeEach(function(){
var todoTexts = ["eat breakfast", "have lunch", "go out to dinner"];
renderer = TestUtils.createRenderer();
renderer.render(
<TodoList todoTexts={ todoTexts } />
);
todolist = renderer.getRenderOutput();
});
it("renders a todolist", function(){
expect(todolist.props.className).to.equal("todo-list");
});
it("greets the user", function(){
var greetingSpan = todolist.props.children[0];
expect(greetingSpan.props.className).to.equal("greeting");
});
it("will render three todos", function(){
var todos = shallowTestUtils.findAllWithType(todolist, Todo);
expect(todos.length).to.equal(3);
var dinnerTodo = todos[2];
expect(dinnerTodo).to.deep.equal(
<Todo key="todo-2" text="go out to dinner" />
);
});
});
Woah, okay, that’s a lot of information. Let’s break it up piece by piece.
Loading Dependencies
The dependencies that I’ve loaded up are as follows:
import chai from "chai";
import jsxChai from "jsx-chai";
import React from "react";
import ReactDOM from "react-dom";
import TestUtils from "react-addons-test-utils";
import shallowTestUtils from "react-shallow-testutils";
import TodoList from "app/components/todolist";
import Todo from "app/components/todo";
chai.use(jsxChai);
var expect = chai.expect;
Let’s go through these piece by piece.
To construct expect statements, I’m using Chai and jsxChai. These libraries will allow us to build expectations directly against JSX components, like expect(component).to.equal(<Todo />)
for example.
As of React 0.14.0, the main functionality of React is split into two dependencies, React
and ReactDOM
. The former consists of all of the fundamental logic that React relies upon, and the latter gives us the virtual DOM into which we can render components and verify that they render the way we want to in the tests.
TestUtils
and shallowTestUtils
are two other optional React libraries provided by Facebook itself. They are going to be the primary libraries which will allow us to artificially render our components and inspect their inner workings.
Finally, we’re testing the TodoList
, so we need that, as well as its inner component, Todo
, which we’ll use in a test later on.
The Setup
beforeEach(function(){
var todoTexts = ["eat breakfast", "have lunch", "go out to dinner"];
renderer = TestUtils.createRenderer();
renderer.render(
<TodoList todoTexts={ todoTexts } />
);
todolist = renderer.getRenderOutput();
});
This beforeEach
simply sets up a suitable test environment for each test. Using TestUtils.createRenderer()
, it generates an object which can superficially render a React component and return that object which is rendered (in this case, todolist
). Since we declared todolist
and renderer
beforehand, we now have access to them in the scopes of the test.
Test 1: The Todolist
it("renders a todolist", function(){
expect(todolist.props.className).to.equal("todo-list");
});
As noted above, the todolist
variable is the object which is to be rendered. We can treat it like other React components and inspect its props - here we’re just making sure that it receives a className
of “todo-list”.
Test 2: The Greeting
it("greets the user", function(){
var greetingSpan = todolist.props.children[0];
expect(greetingSpan.props.className).to.equal("greeting");
});
As this test shows, not only can we inspect the rendered component itself, but its children as well. Since the children are delivered down through React components via props, we can obtain the children there. This test simply grabs the first child of the rendered component and ensures that it has a class of “greeting”.
Test 3: The Todos
it("will render three todos", function(){
var todos = shallowTestUtils.findAllWithType(todolist, Todo);
expect(todos.length).to.equal(3);
var dinnerTodo = todos[2];
expect(dinnerTodo).to.deep.equal(
<Todo key="todo-2" text="go out to dinner" />
);
});
The findAllWithType
method comes in super clutch right here.
It traverses the semi-rendered TodoList
and returns an array of all sub-components that are of the type passed in the second argument. This way, we can make sure that there are exactly as many Todo
s as we expect (3, since we passed 3 in to the TodoList
), and they look like exactly what we expect.
But Wait, Jake! React Doesn’t work like that, does it?
I know what you may be thinking right now: If the renderer
renders a TodoList
that contains Todo
s, shouldn’t the Todo
s be rendered as well? Isn’t this almost a recursive process of rendering down to the smallest, dumbest sub-component?
No, because we’re using shallowTestUtils
! This tool allows us to inspect only the rendered output of that component itself, leaving all React subcomponents as unrendered. In this way, we only need to check that the correct props are passed to subcomponents, and don’t need to worry about their inner details here (we should definitely make a Todo
test though!)
Like, totally cool, right?
In this way, React components are made to be very prone to unit testing - we can test a parent component to make sure it renders the subcomponents it needs to, while not having to look at the entire DOM structure that gets rendered when that component is loaded in the browser. It’s really helpful, especially given the complicated nature of front-end and user-simulating tests. So, with your new knowledge of React’s Shallow Rendering, go forth and test!
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