compilationMode

{
  compilationMode: 'infer' // or 'annotation', 'syntax', 'all'
}

• Reference
  • compilationMode
• Usage
  • Default inference mode
  • Incremental adoption with annotation mode
  • Using Flow syntax mode
  • Opting out specific functions
• Troubleshooting
  • Component not being compiled in infer mode

---

Reference

compilationMode

Controls the strategy for determining which functions the React Compiler will optimize.

Type

'infer' | 'syntax' | 'annotation' | 'all'

Default value

'infer'

Options

• 'infer' (default): The compiler uses intelligent heuristics to identify React components and hooks:

  • Functions explicitly annotated with "use memo" directive
  • Functions that are named like components (PascalCase) or hooks (use prefix) AND create JSX and/or call other hooks

• 'annotation': Only compile functions explicitly marked with the "use memo" directive. Ideal for incremental adoption.

• 'syntax': Only compile components and hooks that use Flow’s component and hook syntax.

• 'all': Compile all top-level functions. Not recommended as it may compile non-React functions.

Caveats

• The 'infer' mode requires functions to follow React naming conventions to be detected
• Using 'all' mode may negatively impact performance by compiling utility functions
• The 'syntax' mode requires Flow and won’t work with TypeScript
• Regardless of mode, functions with "use no memo" directive are always skipped

---

Usage

Default inference mode

The default 'infer' mode works well for most codebases that follow React conventions:

{
  compilationMode: 'infer'
}

With this mode, these functions will be compiled:

// ✅ Compiled: Named like a component + returns JSX
function Button(props) {
  return <button>{props.label}</button>;
}

// ✅ Compiled: Named like a hook + calls hooks
function useCounter() {
  const [count, setCount] = useState(0);
  return [count, setCount];
}

// ✅ Compiled: Explicit directive
function expensiveCalculation(data) {
  "use memo";
  return data.reduce(/* ... */);
}

// ❌ Not compiled: Not a component/hook pattern
function calculateTotal(items) {
  return items.reduce((a, b) => a + b, 0);
}

Incremental adoption with annotation mode

For gradual migration, use 'annotation' mode to only compile marked functions:

{
  compilationMode: 'annotation'
}

Then explicitly mark functions to compile:

// Only this function will be compiled
function ExpensiveList(props) {
  "use memo";
  return (
    <ul>
      {props.items.map(item => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
}

// This won't be compiled without the directive
function NormalComponent(props) {
  return <div>{props.content}</div>;
}

Using Flow syntax mode

If your codebase uses Flow instead of TypeScript:

{
  compilationMode: 'syntax'
}

Then use Flow’s component syntax:

// Compiled: Flow component syntax
component Button(label: string) {
  return <button>{label}</button>;
}

// Compiled: Flow hook syntax
hook useCounter(initial: number) {
  const [count, setCount] = useState(initial);
  return [count, setCount];
}

// Not compiled: Regular function syntax
function helper(data) {
  return process(data);
}

Opting out specific functions

Regardless of compilation mode, use "use no memo" to skip compilation:

function ComponentWithSideEffects() {
  "use no memo"; // Prevent compilation

  // This component has side effects that shouldn't be memoized
  logToAnalytics('component_rendered');

  return <div>Content</div>;
}

---

Troubleshooting

Component not being compiled in infer mode

In 'infer' mode, ensure your component follows React conventions:

// ❌ Won't be compiled: lowercase name
function button(props) {
  return <button>{props.label}</button>;
}

// ✅ Will be compiled: PascalCase name
function Button(props) {
  return <button>{props.label}</button>;
}

// ❌ Won't be compiled: doesn't create JSX or call hooks
function useData() {
  return window.localStorage.getItem('data');
}

// ✅ Will be compiled: calls a hook
function useData() {
  const [data] = useState(() => window.localStorage.getItem('data'));
  return data;
}