leader-effect-captures

Recommended: warn. Flags values declared in a component that a useLeaderEffect callback reads.

Why

useLeaderEffect runs when this tab takes the seat and tears down when it loses it. That is the whole contract, and its dependency list is exactly [isLeader] — deliberately, because depending on the callback’s identity would reconnect an inline arrow’s WebSocket on every render.

The consequence: whatever the effect reads is read at the moment leadership arrived, and nothing re-runs it when that value changes.

function Chat({ roomId }) {
  // ✗ Connected to whichever room was mounted when this tab became leader.
  useLeaderEffect(() => connect(roomId));
}

Navigate to another room and the leader stays on the old socket. Every other tab looks correct — they are not the leader, so they never ran the effect at all — which makes this one of the harder bugs to reproduce.

Correct

A ref is the escape hatch: stable box, current value.

function Chat({ roomId }) {
  const roomRef = useRef(roomId);
  useEffect(() => {
    roomRef.current = roomId;
  });

  useLeaderEffect(() => {
    const socket = connect(roomRef.current);
    return () => socket.close();
  });
}

If the effect genuinely has to restart when the value changes, the value belongs in shared state, or the work belongs in a plain useEffect guarded by useIsLeader() — which does re-run on its own dependencies.

What is exempt

• Anything from module scope or an import: stable by construction.
• useRef results.
• The setter half of useState, useReducer, useSharedState and useSharedReducer — stable by contract.
• The registry getters and channel hooks (useChannel, useSend, useAsk, getSharedStore, getLeader, getChannel): they return the one instance registered for a name.

When not to use it

A value that never changes after mount — a config object built once in the component — is captured harmlessly, and the rule cannot tell. That is why it warns rather than errors. Hoist the value out of the component, or turn the rule off for the file, whichever is more honest.