@dependable/state

Observables and computeds for reactive state management

Easy learning curve
Zero dependencies
Allows multiple versions of the library in the page
Batches updates
TypeScript support

Install

# npm
npm install --save @dependable/state

# yarn
yarn add @dependable/state

Usage

Let's make a small todo application.

First we will import observable and computed, that is used to define the state.

import { observable, computed } from "@dependable/state";

Then we can declare an observable for our todos.

const todos = observable([]);

We will be putting todos inside of the todos observable, so let's define a class for a todo.

As you can see, we defined title and completed as observables, as those fields are allowed to be updated.

let nextId = 0;

class Todo {
  constructor(title) {
    this.id = nextId++;
    const key = `todo.${this.id}`;
    this.title = observable(title);
    this.completed = observable(false);
  }
}

We would like to have a list of active todos, that is sorted by the title.

We can start by creating a sortedTodos computed, that will return all todos sorted by title.

const compareByTitle = (a, b) => {
  if (a.title() > b.title()) return 1;
  if (a.title() < b.title()) return -1;
  return 0;
};

const sortedTodos = computed(() => todos().slice().sort(compareByTitle));

Then we can create another todo, that filters out any completed todos.

const activeTodos = computed(() =>
  sortedTodos().filter((todo) => !todo.completed())
);

Let's add a convenient way of adding new todos to the todos observable.

const addTodo = (title) => {
  todos([...todos(), new Todo(title)]);
};

Now we can add a few todos.

addTodo("Paint the house");
addTodo("Mow the lawn");
addTodo("Buy garden plants");

Finally we need to be able to complete the todos, so let's add a function for that.

const completeTodoByTitle = (title) => {
  const todo = todos().find((todo) => todo.title() === title);
  todo?.completed(true);
  return todo;
};

I already mowed the lawn, so let's complete that task.

completeTodoByTitle("Mow the lawn");

Let's see what our activeTodos contain at this point.

expect(activeTodos(), "to satisfy", [
  { title: "Buy garden plants", completed: false },
  { title: "Paint the house", completed: false },
]);

As you can see it is very easy to build something that is quite powerful.

Subscribing

You can subscribe to both observables and computeds. Updates will be batched and each listener will only be called once for each batch. Listeners will only be called if the value we updated.

const message = observable("");
const aboveLimit = computed(() => message().length > 140);

aboveLimit.subscribe(() => {
  if (aboveLimit()) {
    // show warning
  }
});

Overriding equality comparison for computeds

By default computeds compare values for equality using the Object.is function. The library uses this function to figure out if a computed has changed is value and therefore should notify it's subscribers.

In certain situations it is useful to override this comparison function, that can be done the following way.

const numbers = observable([1, 3, 0, 2]);

const arrayIsEquals = (a, b) => {
  if (a.length !== b.length) return false;

  for (let i = 0; i < a.length; i++) {
    if (a[i] !== b[i]) return false;
  }

  return true;
};

const sortedNumbers = computed(() => numbers().slice().sort(), {
  isEqual: arrayIsEquals,
});

numbers.subscribe(() => {
  console.log(numbers());
});

// This doesn't trigger the subscription
numbers([1, 2, 3, 0]);

// but this does
numbers([1, 2, 3, 0, 4]);

Testing

@dependable/state is build with testing in mind from the beginning. The idea is that you update the observables to the necessary state in the test setup, do some interaction and test the updated state.

We provide a flush method, for synchronously updating the computeds and notifying any listeners, when you need to make sure that derived state is updated.

import { flush } from "@dependable/state";

const numbers = observable([]);
const sum = computed(() => numbers.reduce((sum, v) => sum + v, 0));

let notified = false;
sum.subscribe(() => {
  notified = true;
});

numbers([1, 2, 3]);

// subscribers will be called in the next tick, but we can force the update through
flush();

expect(sum(), "to equal", 6);
expect(notified, "to be true");

Acknowledgements

The observables, computeds and dependency tracking is very inspired by https://knockoutjs.com/

License

MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.