I'm trying to use @testing-library/react waitfor to wait for a lazy loaded component before I compare snapshots.

The App.tsx is simply this:

const MyComponent = lazy(() => import('my/component'));

function App() {


  return (
    <Suspense
      fallback={
        <div>
          <h1>Loading&hellip;</h1>
        </div>
      }
    >
      <MyComponent />
    </Suspense>
  );

MyComponent is:

export const MyComponent = memo(() => {
  return <div data-testid='test' />;
});

And the test is:

describe('<App />', () => {
  it('should render the App', async () => {
    const { container, findByTestId } = render(<App />);
    await waitFor(() => findByTestId('test'));
    expect(container.firstChild).toMatchSnapshot();
  });
});

The waitFor timeout because findByTestId('test') isn't finding the component. If I replace <MyComponent /> with <div data-testid='test' /> then the test works as expected.

Why doesn't render correctly if its being lazily loaded?

Adam

CodePudding user response：

From the React.lazy doc:

React.lazy takes a function that must call a dynamic import(). This must return a Promise which resolves to a module with a default export containing a React component.

This is the signature of the React.lazy method:

function lazy<T extends ComponentType<any>>(
    factory: () => Promise<{ default: T }>
): LazyExoticComponent<T>;

So the factory function should be:

const MyComponent = lazy(() => import('./component').then(({ MyComponent }) => ({ default: MyComponent })));

Besides, you don't need to use waitFor together with findBy* queries, because:

findBy* methods are a combination of getBy* queries and waitFor

The complete working example:

component.tsx:

import React from 'react';
import { memo } from 'react';

export const MyComponent = memo(() => {
  return <div data-testid="test" />;
});

index.tsx:

import React, { lazy, Suspense } from 'react';

const MyComponent = lazy(() => import('./component').then(({ MyComponent }) => ({ default: MyComponent })));

export function App() {
  return (
    <Suspense
      fallback={
        <div>
          <h1>Loading&hellip;</h1>
        </div>
      }
    >
      <MyComponent />
    </Suspense>
  );
}

index.test.tsx:

import { render } from '@testing-library/react';
import '@testing-library/jest-dom/extend-expect';
import React from 'react';
import { App } from '.';

describe('<App />', () => {
  it('should render the App', async () => {
    const { container, findByTestId } = render(<App />);
    expect(container.firstChild).toMatchInlineSnapshot(`
      <div>
        <h1>
          Loading…
        </h1>
      </div>
    `);
    const mycomp = await findByTestId('test');
    expect(mycomp).toBeInTheDocument();
    expect(container.firstChild).toMatchInlineSnapshot(`
      <div
        data-testid="test"
      />
    `);
  });
});

Test result:

 PASS  stackoverflow/71926928/index.test.tsx (11.382 s)
  <App />
    ✓ should render the App (78 ms)

---------------|---------|----------|---------|---------|-------------------
File           | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
---------------|---------|----------|---------|---------|-------------------
All files      |     100 |      100 |     100 |     100 |                   
 component.tsx |     100 |      100 |     100 |     100 |                   
 index.tsx     |     100 |      100 |     100 |     100 |                   
---------------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   2 passed, 2 total
Time:        12.685 s

package versions:

"react": "^16.14.0",
"jest": "^26.6.3",
"@testing-library/react": "^11.2.7"