React Component Profiling Cheatsheet

Note: Ensure you have the React DevTools extension installed in your browser to follow this guide.

Accessing the Profiler

1. Open React DevTools in your browser's developer tools panel.
2. Click on the "Profiler" tab in the toolbar.

Starting and Stopping the Profiling Session

1. Click the "Record" button to start a profiling session.
2. Interact with your app as you normally would. The Profiler will collect performance information during this time.
3. Click the "Record" button again to stop the profiling session.

Analyzing the Results

1. Flamegraph: The Flamegraph represents each component that was rendered during the profiling session. Each bar represents a React component (the wider the bar, the longer it took to render).

2. Ranked Chart: The ranked chart presents the same information as the flame graph, but in a list form sorted by render time.

3. Component Chart: The component chart shows how many times each component rendered during the profiling session.

Understanding the Flamegraph

1. Color: Each bar in the flamegraph is color-coded to represent how much time was spent rendering the components:

   • Gray: time spent rendering components outside of the profiling session.
   • Blue: time spent on the current commit where components were rendered.
   • Yellow: time spent on other activities.

2. Width: The width of a bar represents how long the component (and its children) took to render. A wider bar means a longer render time.

Selecting a Commit

1. Use the slider or the arrows at the bottom to select a specific commit to inspect.

Inspecting a Component

1. Click on a bar in the flamegraph to see more information about the component. This will show:
   • Which props changed since the last render.
   • The actual and base durations of the component.

Analyzing the Results

1. Long Bars in the Flamegraph: Look for bars that are significantly wider than others. These components took longer to render and might be candidates for optimization.

2. Frequent Re-renders in the Component Chart: If a component shows a high count in the component chart, it might be rendering too often.

Profiling Options

1. Record why each component rendered while profiling: This option can provide more information about why a component re-rendered.

2. Highlight updates when components render: This can be useful to visually see which parts of the page are re-rendering.

Case Study 1: Profiling a Slow List Component

Imagine you have a ProductList component that renders a large list of ProductItem components. You've noticed that when a single product is added or removed, the entire list re-renders, causing a noticeable delay.

Step 1: Start Profiling

Start the profiling session and interact with the ProductList component.

Step 2: Analyze the Flamegraph

In the Flamegraph, you notice that ProductList is taking a considerable amount of time to render, and every ProductItem is also being re-rendered.

Step 3: Inspect the Component

By selecting the ProductList component in the flamegraph, you find that the products prop is changing every time, causing the component and all its children to re-render.

Solution

In this case, the issue might be that a new products array is created every time the state changes. A possible solution would be to ensure that product items have consistent identities across renders. You could use the useMemo hook to memoize the products array, or ensure that the ProductItem components are only re-rendering when their specific product changes.

Case Study 2: Profiling a Complex Form Component

Let's say you have a UserForm component with several input fields. Users have reported that typing into the fields feels sluggish.

Step 1: Start Profiling

Begin the profiling session, then interact with the form by typing into the fields.

Step 2: Analyze the Component Chart

The Component Chart reveals that the UserForm component is re-rendering every time you type a character into any field.

Step 3: Inspect the Component

Upon selecting the UserForm in the flamegraph, you find that the entire form re-renders every time the formValues state changes.

Solution

The issue could be that the form is keeping all of its values in a single state object, causing a re-render every time any field changes. A possible solution would be to break up the formValues state into individual state variables for each field using the useState hook. This way, changing the state of one field doesn't cause the others to re-render.

In both these cases, using React DevTools Profiler allowed us to identify performance issues and helped us strategize the solutions to enhance the app's performance.

Remember, the goal of profiling is not to eliminate all re-renders or to make your flame graph perfectly flat. Instead, use these tools to find the areas of your app that are doing more work than necessary, and optimize accordingly.