如何测试在componentDidMount中设置异步调用以设置React Component的状态

Question

What is the best way to test that an async call within componentDidMount sets the state for a React component? For context, the libraries I'm using for testing are Mocha , Chai , Enzyme , and Sinon .

Here's an example code:

/* 
 * assume a record looks like this:
 * { id: number, name: string, utility: number }
 */

// asyncComponent.js
class AsyncComponent extends React.Component {
    constructor(props) {
        super(props);

        this.state = {
            records: []
        };
    }

    componentDidMount() {
        // assume that I'm using a library like `superagent` to make ajax calls that returns Promises

        request.get('/some/url/that/returns/my/data').then((data) => {
            this.setState({
                records: data.records
            });
        });
    }

    render() {
        return (
            <div className="async_component">
                { this._renderList() }
            </div>
        );
    }

    _renderList() {
        return this.state.records.map((record) => {
            return (
                <div className="record">
                    <p>{ record.name }</p>
                    <p>{ record.utility }</p>
                </div>
            );
        });
    }
}


// asyncComponentTests.js
describe("Async Component Tests", () => {
    it("should render correctly after setState in componentDidMount executes", () => {
        // I'm thinking of using a library like `nock` to mock the http request

       nock("http://some.url.com")
           .get("/some/url/that/returns/my/data")
           .reply(200, {
               data: [
                   { id: 1, name: "willson", utility: 88 },
                   { id: 2, name: "jeffrey", utility: 102 }
               ]
           });

       const wrapper = mount(<AsyncComponent />);

       // NOW WHAT? This is where I'm stuck.
    });
});

Answer 1

So, what you are really trying to test is that based on some mock data it "should render correctly ..." .

As some people pointed out, a good way to achieve that is by placing the data fetching logic into a separate container and have a "dumb" presentation component that only knows how to render props .

Here is how to do that: (I had to modify it a bit for Typescript with Tslint, but you'll get the idea)

export interface Props {
    // tslint:disable-next-line:no-any
    records: Array<any>;
}

// "dumb" Component that converts props into presentation 
class MyComponent extends React.Component<Props> {
    // tslint:disable-next-line:no-any
    constructor(props: Props) {
        super(props);
    }

    render() {
        return (
            <div className="async_component">
                {this._renderList()}
            </div>
        );
    }

    _renderList() {
        // tslint:disable-next-line:no-any
        return this.props.records.map((record: any) => {
            return (
                <div className="record" key={record.name}>
                    <p>{record.name}</p>
                    <p>{record.utility}</p>
                </div>
            );
        });
    }
}

// Container class with the async data loading
class MyAsyncContainer extends React.Component<{}, Props> {

    constructor(props: Props) {
        super(props);

        this.state = {
            records: []
        };
    }

    componentDidMount() {

        fetch('/some/url/that/returns/my/data')
        .then((response) => response.json())
        .then((data) => {
            this.setState({
                records: data.records
            });
        });
    }

    // render the "dumb" component and set its props
    render() {
        return (<MyComponent records={this.state.records}/>);
    }
}

Now you can test MyComponent rendering by giving your mock data as props.

Answer 2

Ignoring the, sane, advice to think again about the structure, one way to go about this could be:

• Mock the request (fx with sinon), to make it return a promise for some records
• use Enzyme's mount function
• Assert that the state to not have your records yet
• Have your rest function use done callback
• Wait a bit (fx with setImmediate ), this will make sure your promise is resolved
• Assert on the mounted component again, this time checking that the state was set
• Call your done callback to notify that the test has completed

So, in short:

// asyncComponentTests.js
describe("Async Component Tests", () => {
    it("should render correctly after setState in componentDidMount executes", (done) => {
        nock("http://some.url.com")
           .get("/some/url/that/returns/my/data")
           .reply(200, {
               data: [
                   { id: 1, name: "willson", utility: 88 },
                   { id: 2, name: "jeffrey", utility: 102 }
               ]
           });

        const wrapper = mount(<AsyncComponent />);

        // make sure state isn't there yet
        expect(wrapper.state).to.deep.equal({});

        // wait one tick for the promise to resolve
        setImmediate(() => {
            expect(wrapper.state).do.deep.equal({ .. the expected state });
            done();
        });
    });
});

Note:

I have no clue about nock, so here I assume your code is correct

Answer 3

IMO, this is actually a common issue which appears more complicated because of promises and componentDidMount : You're trying to test a functions which are only defined within the scope of another function. ie You should split your functions out and test them individually.

Component

class AsyncComponent extends React.Component {
    constructor(props) {
        super(props);

        this.state = {
            records: []
        };
    }

    componentDidMount() {
        request.get('/some/url/that/returns/my/data')
            .then(this._populateState);
    }

    render() {
        return (
            <div className="async_component">
                { this._renderList() }
            </div>
        );
    }

    _populateState(data) {
        this.setState({
            records: data.records
        });
    }

    _renderList() {
        return this.state.records.map((record) => {
            return (
                <div className="record">
                    <p>{ record.name }</p>
                    <p>{ record.utility }</p>
                </div>
            );
        });
    }
}

Unit Test

// asyncComponentTests.js
describe("Async Component Tests", () => {
    describe("componentDidMount()", () => {
        it("should GET the user data on componentDidMount", () => {
            const data = {
                records: [
                    { id: 1, name: "willson", utility: 88 },
                    { id: 2, name: "jeffrey", utility: 102 }
                ]
            };
            const requestStub = sinon.stub(request, 'get').resolves(data);
            sinon.spy(AsyncComponent.prototype, "_populateState");
            mount(<AsyncComponent />);

            assert(requestStub.calledOnce);
            assert(AsyncComponent.prototype._populateState.calledWith(data));
        });
    });

    describe("_populateState()", () => {
        it("should populate the state with user data returned from the GET", () => {
            const data = [
                { id: 1, name: "willson", utility: 88 },
                { id: 2, name: "jeffrey", utility: 102 }
            ];

            const wrapper = shallow(<AsyncComponent />);
            wrapper._populateState(data);

            expect(wrapper.state).to.deep.equal(data);
        });
    });
});

Note : I've written the unit tests from documentation alone, so the use of shallow , mount , assert , and expect might not be best practices.