zustand-querystring

Zustand middleware for URL query string sync.

npm install zustand-querystring

Usage

import { create } from 'zustand';
import { querystring } from 'zustand-querystring';

const useStore = create(
  querystring(
    (set) => ({
      search: '',
      page: 1,
      setSearch: (search) => set({ search }),
      setPage: (page) => set({ page }),
    }),
    {
      select: () => ({ search: true, page: true }),
    }
  )
);
// URL: ?search=hello&page=2

---

Options

querystring(storeCreator, {
  select: undefined,    // which fields to sync
  key: false,           // false | 'state'
  prefix: '',           // prefix for URL params
  format: marked,       // serialization format
  syncNull: false,      // sync null values
  syncUndefined: false, // sync undefined values
  url: undefined,       // request URL for SSR
})

select

Controls which state fields sync to URL. Receives pathname, returns object with true for fields to sync.

// All fields
select: () => ({ search: true, page: true, filters: true })

// Route-based
select: (pathname) => ({
  search: true,
  filters: pathname.startsWith('/products'),
  adminSettings: pathname.startsWith('/admin'),
})

// Nested fields
select: () => ({
  user: {
    name: true,
    settings: { theme: true },
  },
})

key

• false (default): Each field becomes a separate URL param

  ?search=hello&page=2&filters.sort=name
  

• 'state' (or any string): All state in one param

  ?state=search%3Dhello%2Cpage%3A2
  

prefix

Adds prefix to all params. Use when multiple stores share URL.

querystring(storeA, { prefix: 'a_', select: () => ({ search: true }) })
querystring(storeB, { prefix: 'b_', select: () => ({ filter: true }) })
// URL: ?a_search=hello&b_filter=active

syncNull / syncUndefined

By default, null and undefined reset to initial state (removed from URL). Set to true to write them.

url

For SSR, pass the request URL:

querystring(store, { url: request.url, select: () => ({ search: true }) })

---

How State Syncs

1. On page load: URL → State
2. On state change: State → URL (via replaceState)

Only values different from initial state are written to URL:

// Initial: { search: '', page: 1, sort: 'date' }
// Current: { search: 'hello', page: 1, sort: 'name' }
// URL: ?search=hello&sort=name
// (page omitted - matches initial)

Type handling:

• Objects: recursively diffed
• Arrays, Dates: compared as whole values
• Functions: never synced

---

Formats

Three built-in formats:

Format	Example Output
marked	count:5,tags@a,b~
plain	count=5&tags=a,b
json	count=5&tags=%5B%22a%22%5D

import { marked } from 'zustand-querystring/format/marked';
import { plain } from 'zustand-querystring/format/plain';
import { json } from 'zustand-querystring/format/json';

querystring(store, { format: plain })

Marked Format (default)

Type markers: : primitive, = string, @ array, . object

Delimiters: , separator, ~ terminator, _ escape

import { createFormat } from 'zustand-querystring/format/marked';

const format = createFormat({
  typeObject: '.',
  typeArray: '@',
  typeString: '=',
  typePrimitive: ':',
  separator: ',',
  terminator: '~',
  escapeChar: '_',
  datePrefix: 'D',
});

Plain Format

Dot notation for nesting, comma-separated arrays.

import { createFormat } from 'zustand-querystring/format/plain';

const format = createFormat({
  entrySeparator: ',',           // between entries in namespaced mode
  nestingSeparator: '.',         // for nested keys
  arraySeparator: ',',           // or 'repeat' for ?tags=a&tags=b&tags=c
  escapeChar: '_',
  nullString: 'null',
  undefinedString: 'undefined',
  infinityString: 'Infinity',    // string representation of Infinity
  negativeInfinityString: '-Infinity',
  nanString: 'NaN',
});

JSON Format

URL-encoded JSON. No configuration.

---

Custom Format

Implement QueryStringFormat:

import type { QueryStringFormat, QueryStringParams, ParseContext } from 'zustand-querystring';

const myFormat: QueryStringFormat = {
  // For key: 'state' (namespaced mode)
  stringify(state: object): string {
    return encodeURIComponent(JSON.stringify(state));
  },
  parse(value: string, ctx?: ParseContext): object {
    return JSON.parse(decodeURIComponent(value));
  },

  // For key: false (standalone mode)
  stringifyStandalone(state: object): QueryStringParams {
    const result: QueryStringParams = {};
    for (const [key, value] of Object.entries(state)) {
      result[key] = [encodeURIComponent(JSON.stringify(value))];
    }
    return result;
  },
  parseStandalone(params: QueryStringParams, ctx: ParseContext): object {
    const result: Record<string, unknown> = {};
    for (const [key, values] of Object.entries(params)) {
      result[key] = JSON.parse(decodeURIComponent(values[0]));
    }
    return result;
  },
};

querystring(store, { format: myFormat })

Types:

• QueryStringParams = Record<string, string[]> (values always arrays)
• ctx.initialState available for type coercion

---

Examples

Search with reset

const useStore = create(
  querystring(
    (set) => ({
      query: '',
      page: 1,
      setQuery: (query) => set({ query, page: 1 }), // reset page on new query
      setPage: (page) => set({ page }),
    }),
    { select: () => ({ query: true, page: true }) }
  )
);

Multiple stores with prefixes

const useFilters = create(
  querystring(filtersStore, {
    prefix: 'f_',
    select: () => ({ category: true, price: true }),
  })
);

const usePagination = create(
  querystring(paginationStore, {
    prefix: 'p_',
    select: () => ({ page: true, limit: true }),
  })
);
// URL: ?f_category=shoes&f_price=100&p_page=2&p_limit=20

Next.js SSR

// app/page.tsx
export default async function Page({ searchParams }) {
  // Store reads from URL on init
}

---

Exports

// Middleware
import { querystring } from 'zustand-querystring';

// Formats
import { marked, createFormat } from 'zustand-querystring/format/marked';
import { plain, createFormat } from 'zustand-querystring/format/plain';
import { json } from 'zustand-querystring/format/json';

// Types
import type {
  QueryStringOptions,
  QueryStringFormat,
  QueryStringParams,
  ParseContext,
} from 'zustand-querystring';

---

Playground · GitHub