js-atom

Clojure(Script) atoms in plain JavaScript.

Why?

Immutable values require some sort of external state management in order for your app to change state. One option is to use variables in scope to hold the current state of your app:

function startApp(root) {
    var data;
 
    function render() {
        React.renderComponent(AppUI(data), root);
    }
 
    pollForData(function (newData) {
        data = newData;
        render();
    });
 
    // ...
}

This sort of works, but quickly becomes unwieldy. Atoms offer a formal mechanism for maintaining a single mutable reference. An atom is used to hold the current state. It offers a small API for changing (or replacing) this state, subscribing to changes, and for validating state changes.

function startApp(root) {
    var state = atom.createAtom({});
 
    function render() {
        React.renderComponent(AppUI(state.deref()), root);
    }
 
    // Render when the state changes
    state.addWatch("poll-update", render);
 
    pollForNewData(function (newData) {
        state.replace(newData);
    });
 
    // ...
}

Immutability

Atoms are most useful when containing immutable values, but there's nothing stopping you from sticking whatever you want in them. If you put a mutable value in the atom, you either have to make sure you don't actually mutate it, or lose some of the benefits (e.g. being able to trust past versions of the state).

API

The API is designed to mirror Clojure's atoms as closely as possible. Because atoms are references, and not values, I didn't see any problems with defining the API as methods on the atom object.

createAtom(val[, options])

Creates a new atom wrapping the provided value. options is optional, and currently only supports one option: validator:

var createAtom = require("js-atom");
var atom = createAtom([], { validator: Array.isArray });
 
atom.replace([1, 2, 3]); // OK
atom.replace({}); // Throws exception

atom.deref()

Returns the contained value.

atom.reset(val)

Replace the current state with a new value

atom.swap(fn[, ...])

Update the state by applying the function to the current value, and setting the return value as the new value of the atom. Any additional arguments are passed to the function as well, after the atom value, e.g.: atom.swap(fn, 1, 2, 3) will replace the current value with what is returned from fn(atomValue, 1, 2, 3).

atom.addWatch(key, function (key, ref, old, new) {})

Add a function that will be called whenever the atom value changes. The key is just a string identifying this watcher - it can be used to remove the watcher again. The callback is called with four arguments whenever the state changes (e.g. with reset or swap):

• key - The key used to register the watcher
• ref - The atom reference
• old - The previous value
• new - The new value

removeWatch(key)

Removes the previously added watcher.

toString

Prints a useful string representation of the contents of the atom.

License

Copyright © 2014, Christian Johansen. js-atom uses semantic versioning. Code released under the BSD license. Documentation released under CC Attribution-Share Alike.