State

setState replaces state. It does not merge. That is the 2.x contract.

this.setState({ clicks: this.state.clicks + 1 });
this.setState((s) => ({ ...s, clicks: s.clicks + 1 }));

Several setState calls in one event are batched into a single microtask patch.

In development the new state is structuredCloned and deep-frozen. Mutating this.state.clicks++ throws. In production the clone still happens; the freeze does not.

State must be structured-cloneable. Functions, DOM nodes, and similar runtime objects belong on the component instance (this.timer, this.element), not inside this.state; SvenJS throws a clear error instead of silently dropping them.

initialState may be a value or a factory that receives props. A factory is useful when each mount needs state derived from its own props, and keeps server rendering and hydration deterministic.

const Counter = create({
  initialState: (props) => ({ count: props.start ?? 0 }),
  render() {
    return <button>{this.state.count}</button>;
  },
});

Pass a full object. If you need fields from the previous state, copy them — or use the updater form.