ReadonlyAtom

The ReadonlyAtom type represents a read-only reactive state container

ReadonlyAtom

The ReadonlyAtom<T> type represents a read-only reactive state container that can be read and subscribed to, but cannot be directly modified.

Type Definition

/**
 * An atom that is read-only and cannot be set.
 *
 * @example
 *
 * ```ts
 * const atom = createAtom(() => 42);
 * // @ts-expect-error - Cannot set a readonly atom
 * atom.set(43);
 * ```
 */
interface ReadonlyAtom<T> extends BaseAtom<T> {}

interface BaseAtom<T> extends Subscribable<T>, Readable<T> {}

Properties

get

Inherited from Readable<T>. Returns the current value of the atom.

Type:

() => T

subscribe

Inherited from Subscribable<T>. Subscribes to changes in the atom’s value.

Type:

((observer: Observer<T>) => Subscription) &
  ((
    next: (value: T) => void,
    error?: (error: any) => void,
    complete?: () => void,
  ) => Subscription)

Usage Examples

Creating a readonly atom

import { atom } from '@tanstack/store'

const writableAtom = atom(42)
const readonlyAtom: ReadonlyAtom<number> = writableAtom

// Reading is allowed
console.log(readonlyAtom.get()) // 42

// Setting is not allowed (TypeScript error)
// readonlyAtom.set(43) // Error: Property 'set' does not exist

Derived readonly atoms

import { atom } from '@tanstack/store'

const countAtom = atom(0)
const doubleCountAtom = atom((get) => get(countAtom) * 2)

// doubleCountAtom is effectively readonly
console.log(doubleCountAtom.get()) // 0

countAtom.set(5)
console.log(doubleCountAtom.get()) // 10

// Cannot directly set a derived atom
// doubleCountAtom.set(20) // Error if typed as ReadonlyAtom

Subscribing to readonly atoms

import { atom } from '@tanstack/store'

const sourceAtom = atom(1)
const readonlyAtom: ReadonlyAtom<number> = sourceAtom

// Subscribe to changes
const subscription = readonlyAtom.subscribe((value) => {
  console.log('Value changed:', value)
})

// Changes to the source are reflected
sourceAtom.set(2) // Logs: "Value changed: 2"

subscription.unsubscribe()

Using as function parameter

import { atom, type ReadonlyAtom } from '@tanstack/store'

function useAtomValue<T>(atom: ReadonlyAtom<T>): T {
  // Function only needs to read, not write
  return atom.get()
}

const myAtom = atom(42)
const value = useAtomValue(myAtom) // Works with both Atom and ReadonlyAtom
console.log(value) // 42

When to Use

ReadonlyAtom<T> is useful when:

• You want to expose an atom for reading without allowing external modifications
• Creating derived/computed atoms that depend on other atoms
• Enforcing read-only access in function parameters or return types
• Building public APIs where internal state should not be directly modified

Related Types

• Atom - The writable version with a set method
• Observer - The observer pattern for subscriptions
• Subscription - The subscription object returned by subscribe
• AtomOptions - Configuration options for creating atoms