React Performance Testing with Jest

Technically, this is already enough to count renders. But we wanted to make profiler really easy to use, so we created a Higher-Order Component:

const withProfiler = (Component, id: string = 'rootComponent') => {
  const componentId = `withProfiler${id}`;

  const onRender = () => {
    SnapshotProfiler.__numRenders++;
  };

  const SnapshotProfiler = React.memo((props) => {
    return (
      
        
      
    );
  });

  SnapshotProfiler.__numRenders = 0;

  SnapshotProfiler.clearCounters = function () {
    this.__numRenders = 0;
  };

  return SnapshotProfiler;
};

const Foo = () => (
Foo
)
const DecoratedFoo = withProfiler(Foo);

Now we can just wrap any component with this HOC, and we can access the number of times that component has been rendered. Rendering DecoratedFoo will effectively render Foo, but will also store a __numRenders counter. And since it is stored inside the component object, rendering any instance of this component will increase the counter. So, if we render the following jsx:

we will have DecoratedFoo.__numRenders === 2 .

This is all well and good, but it's still a little awkward to have withProfiler HOC to components used in a real app. Fortunately, Jest has flexible tools for mocking, so we could make great use of them.

Let’s say we have a ChessBoard component that renders 64 instances of Square component. On each move it should typically re-render 2 more cells, since a single chess piece moves from square to square. So we can create the following mock setup and write the following tests:

jest.mock("./Square", () => ({
 Square: jest
   .requireActual("../profiler")
   .withProfiler(jest.requireActual("./Square").Square),
}));
describe("Chess performance test", () => {
 function clearCounters() {
     Square.clearCounters();
 }
 beforeEach(() => {
   clearCounters();
 });
 const initialSquareRendersCount = 64;
 test("initial render", () => {
   render();
   expect(Square.__numRenders).toBe(initialCellRendersCount);
 });
 test("piece moving", () => {
   const { rerender } = render();
   clearCounters();
   //
   // here we do some DOM manipulations or push new props to initiate re-render...
   //
   expect(Square.__numRenders).toBe(2);
 });
})

This way we can be sure the chessboard elements are rendered as many times as we need. So if someone accidentally puts a global state in every cell and the entire board will unnecessarily re-render - we will find out.

What we’ve done here is mocked Square, so in tests we have the same component wrapped into withProfiler HOC. Thus, the tests themselves do not alter the component structure, and are rendered just like in a regular environment. The code of the component itself does not change too. So, if we have a complex hierarchy of components and some really specific actions causing too many re-renders of a particular component, we can test just that.

I hope this helps you write more performant web applications. Good luck!