React useState

React requires a new object reference on every state update. For deeply nested state, this means spreading every intermediate object by hand. data-path replaces manual spreading with a single .set() or .update() call.

Basic usage

import { useState } from "react";
import { path } from "data-path";

type AppState = {
  user: {
    profile: { firstName: string; theme: "light" | "dark" };
  };
};

const firstNamePath = path((s: AppState) => s.user.profile.firstName);
const themePath = path((s: AppState) => s.user.profile.theme);

function ProfileSettings() {
  const [state, setState] = useState<AppState>({
    user: { profile: { firstName: "Alice", theme: "light" } },
  });

  const setFirstName = (name: string) =>
    setState((prev) => firstNamePath.set(prev, name));

  const toggleTheme = () =>
    setState((prev) =>
      themePath.update(prev, (t) => (t === "light" ? "dark" : "light"))
    );

  return (
    <div>
      <input
        value={state.user.profile.firstName}
        onChange={(e) => setFirstName(e.target.value)}
      />
      <button onClick={toggleTheme}>
        Theme: {state.user.profile.theme}
      </button>
    </div>
  );
}

Compared to manual spreading

// Before — spreading each level
setState((prev) => ({
  ...prev,
  user: {
    ...prev.user,
    profile: { ...prev.user.profile, theme: "dark" },
  },
}));

// After — structural clone handled automatically
setState((prev) => themePath.set(prev, "dark"));

The behavior is identical: path.set() returns a structural clone. React detects the new reference and re-renders as expected.

Array items

Capture the index at the call site and build the path dynamically:

type ListState = { items: Array<{ label: string; done: boolean }> };

const labelPath = (i: number) =>
  path((s: ListState) => s.items[i].label);

const donePath = (i: number) =>
  path((s: ListState) => s.items[i].done);

function TodoList() {
  const [state, setState] = useState<ListState>({ items: [] });

  const toggle = (i: number) =>
    setState((prev) =>
      donePath(i).update(prev, (v) => !v)
    );

  return (
    <ul>
      {state.items.map((item, i) => (
        <li key={i} onClick={() => toggle(i)}>
          {item.label}
        </li>
      ))}
    </ul>
  );
}

Defining paths at module level

Paths that don’t depend on runtime indices can be defined outside the component — they are plain values with no lifecycle:

// Module level — created once, stable across renders
const themePath = path((s: AppState) => s.user.profile.theme);

function ThemeToggle() {
  const [state, setState] = useState<AppState>(defaultState);
  const toggle = () =>
    setState((prev) =>
      themePath.update(prev, (t) => (t === "light" ? "dark" : "light"))
    );
  // ...
}

useReducer

The same pattern applies with useReducer — pass the path and value through the action:

type Action =
  | { type: "SET_THEME"; theme: "light" | "dark" }
  | { type: "SET_NAME"; name: string };

function reducer(state: AppState, action: Action): AppState {
  switch (action.type) {
    case "SET_THEME":
      return themePath.set(state, action.theme);
    case "SET_NAME":
      return firstNamePath.set(state, action.name);
    default:
      return state;
  }
}

See also

• Data access — set, update, and immutability details
• Zustand — the same pattern for global stores