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This project is part of the @thi.ng/umbrella monorepo.


Clojure inspired mutable wrappers for (usually) immutable values, with infrastructure support for:

  • watches
  • derived view subscriptions
  • cursors (direct R/W access to nested values)
  • undo/redo history

Together these types act as building blocks for various application state handling patterns, specifically aimed (though not exclusively) at the concept of using a nested, immutable, centralized atom as single source of truth within an application.


Stable, used in production and in active development.

Note: On 2018-03-17 this package was split to remain more focused. Path based getters/setters have been moved into the new @thi.ng/paths package. Likewise, all interceptor based event handling functionality now lives in the @thi.ng/interceptors package.


yarn add @thi.ng/atom

New since 2018-03-15: You can now create a preconfigured app skeleton using @thi.ng/atom, @thi.ng/hdom & @thi.ng/router using the create-hdom-app project generator:

yarn create hdom-app my-app

cd my-app
yarn install
yarn start


Usage examples

Several projects in the /examples directory make heavy use of this library.


An Atom is a mutable wrapper for immutable values. The wrapped value can be obtained via deref(), replaced via reset() and updated using swap(). An atom too supports the concept of watches, essentially onchange event handlers which are called from reset/swap and receive both the old and new atom values.

import * as atom from "@thi.ng/atom";

const a = new atom.Atom(23);

// obtain value via deref()
// 23

// add watch to observe value changes
a.addWatch("foo", (id, prev, curr) => console.log(`${id}: ${prev} -> ${curr}`));
// true

// example update function
const add = (x, y) => x + y;

// apply given function to current value
// (incl. any additional arguments passed to swap)
// this is the same as:
// a.reset(adder(a.deref(), 1))
a.swap(add, 1);
// foo: 23 -> 24

// reset atom's value
// foo: 24 -> 42


Cursors provide direct & immutable access to a nested value within a structured atom. The path to the desired value must be provided when the cursor is created and cannot be changed later. The path is then compiled into a getter and setter to allow cursors to be used like atoms and update the parent state in an immutable manner (i.e. producing an optimized copy with structural sharing of the original (as much as possible)) - see further details below.

It's important to remember that cursors also cause their parent state (atom or another cursor) to reflect their updated local state. I.e. any change to a cursor's value propagates up the hierarchy of parent states.

a = new atom.Atom({a: {b: {c: 1}}})
// cursor to `b` value
b=new atom.Cursor(a, "a.b")
// cursor to `c` value, relative to `b`
c=new atom.Cursor(b, "c")


// { c: 2 }

// { a: { b: { c: 2 } } }

For that reason, it's recommended to design the overall data layout rather wide than deep (my personal limit is 3-4 levels) to minimize the length of the propagation chain and maximize structural sharing.

// main state
main = new atom.Atom({ a: { b: { c: 23 }, d: { e: 42 } }, f: 66 });

// cursor to `c` value
cursor = new atom.Cursor(main, "a.b.c");
// or
cursor = new atom.Cursor(main, ["a","b","c"]);

// alternatively provide path implicitly via lookup & update functions
// both fns will be called with cursor's parent state
// this allows the cursor implementation to work with any data structure
// as long as the updater DOES NOT mutate in place
cursor = new atom.Cursor(
    (s) => s.a.b.c,
    (s, x) => ({...s, a: {...s.a, b: {...s.a.b, c: x}}})

// add watch just as with Atom
cursor.addWatch("foo", console.log);

// 23

cursor.swap(x => x + 1);
// foo 23 24

// { a: { b: { c: 24 }, d: { e: 42 } }, f: 66 }

Derived views

Whereas cursors provide read/write access to nested key paths within a state atom, there are many situations when one only requires read access and the ability to (optionally) produce transformed versions of such a value. The View type provides exactly this functionality:

db = new atom.Atom({a: 1, b: {c: 2}});

// create a view for a's value
viewA = db.addView("a");

// create a view for c's value w/ transformer
viewC = db.addView("b.c", (x) => x * 10);

// 1

// 20

// update the atom
db.swap((state) => atom.setIn(state, "b.c", 3))

// views can indicate if their value has changed
// (will be reset to `false` after each deref)

// here viewA hasn't changed (we only updated `c`)
// false
// true

// the transformer function is only executed once per value change
// 30

// just returns current cached transformed value
// 30

// discard views

Since v1.1.0 views can also be configured to be eager, instead of the "lazy" default behavior. If the optional lazy arg is true (default), the view's transformer will only be executed with the first deref() after each value change. If lazy is false, the transformer function will be executed immediately after a value change occurred and so can be used like a selective watch which only triggers if there was an actual value change (in contrast to normal watches, which execute with each update, regardless of value change).

Related, the actual value change predicate can be customized. If not given, the default `@thi.ng/equiv` will be used.

let x;
let a = new Atom({value: 1})

// create an eager view by passing `false` as last arg
view = a.addView("value", (y) => (x = y, y * 10), false);

// check `x` to verify that transformer already has run
x === 1
// true

// reset x
x = null

// verify transformed value
view.deref() === 10
// true

// verify transformer hasn't rerun because of deref()
x === null
// true

Atoms & views are useful tools for keeping state outside UI components. Here's an example of a tiny @thi.ng/hdom web app, demonstrating how to use derived views to switch the UI for different application states / modules.

Note: The constrained nature of this next example doesn't really do justice to the powerful nature of the approach. Also stylistically, in a larger app we'd want to avoid the use of global variables (apart from db) as done here...

For a more advanced / realworld usage pattern, check the related event handling package and bundled examples.

This example is also available in standalone form:

Source | Live demo

import { Atom, setIn } from "@thi.ng/atom";
import { start } from "@thi.ng/hdom";

// central immutable app state
const db = new Atom({ state: "login" });

// define views for different state values
const appState = db.addView<string>("state");
const error = db.addView<string>("error");
// specify a view transformer for the username value
const user = db.addView<string>(
    (x) => x ? x.charAt(0).toUpperCase() + x.substr(1) : null

// state update functions
const setValue = (path, val) => db.swap((state) => setIn(state, path, val));
const setState = (s) => setValue(appState.path, s);
const setError = (err) => setValue(error.path, err);
const setUser = (e) => setValue(user.path, e.target.value);
const loginUser = () => {
    if (user.deref() && user.deref().toLowerCase() === "admin") {
    } else {
        setError("sorry, wrong username (try 'admin')");
const logoutUser = () => {
    setValue(user.path, null);

// components for different app states
// note how the value views are used here
const uiViews = {
    // dummy login form
    login: () =>
            ["h1", "Login"],
            error.deref() ? ["div.error", error.deref()] : undefined,
            ["input", { type: "text", onchange: setUser }],
            ["button", { onclick: loginUser }, "Login"]
    logout: () =>
            ["h1", "Good bye"],
            "You've been logged out. ",
                { href: "#", onclick: () => setState("login") },
                "Log back in?"
    main: () =>
            ["h1", `Welcome, ${user.deref()}!`],
            ["div", "Current app state:"],
                    { cols: 40, rows: 10 },
                    JSON.stringify(db.deref(), null, 2)]],
            ["button", { onclick: logoutUser }, "Logout"]

// finally define another derived view for the app state value
// including a transformer, which maps the current app state value
// to its correct UI component (incl. a fallback for illegal app states)
const currView = db.addView(
    (state) =>
        uiViews[state] ||
        ["div", ["h1", `No component for state: ${state}`]]

// app root component
const app = () =>
        ["footer", "Made with @thi.ng/atom and @thi.ng/hdom"]];

start(document.body, app);

Undo history

The History type can be used with & behaves like an Atom or Cursor, but creates snapshots of the current state before applying the new state. By default history has length of 100 steps, but this is configurable.

db = new atom.History(new atom.Atom({a: 1}))
// {a: 1}

db.reset({a: 2, b: 3})
db.reset({b: 4})

// {a: 2, b: 3}

// {a: 1}

// undefined (no more undo possible)
// false

// {a: 2, b: 3}

// {b: 4}

// undefined (no more redo possible)

// false


  • Karsten Schmidt


© 2018 Karsten Schmidt // Apache Software License 2.0

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