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Turning properties into objects

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import { box } from "boxm"

const person = new Person();

// get a reference to the firstName property
const firstNameProperty = box(person).firstName;

// get/set the property value
const oldName = firstNameProperty.get();

// grab several references in one hit (destructuring)
const { firstName, lastName, dateOfBirth } = box(person);


npm install --save boxm

(Type declarations are included of course.)


Although it is useful with React and MobX (and the name is a play on MobX, as well as box 'em), boxm has no runtime package dependencies itself.


This library provides a way to conveniently grab a "reference" to a mutable property, with static type safety for TypeScript users.

Properties are named features of objects and so to address one for reading/writing you need to know the object and the name of the property. This can lead to ugly bifurcation and the use of "stringly typed" interfaces.

It's better to make an object with a static get/set interface that encapsulates one property:

export interface BoxedValue<T> { 
    get(): T;
    set(v: T): void;

This can then be passed around as a first-class value. All that's needed is a succinct way to create such an object.

More background here

Fun, Exciting Use-case

In MobX you typically have an object holding the current state of the UI (or a piece of the UI):

class Person {
    @observable firstName = "Leia";
    @observable lastName = "Organa";

And in React you can create modular components. These might be as elemental as a single text input field:

interface TextInputProps {
    text: BoxedValue<string>;  // see BoxedValue<T> declaration above

function TextInput(props: TextInputProps) {
    return <input type="text"
        onChange={e => props.text.set(( as HTMLInputElement).value)}

With such a component you can describe the UI:

const { firstName, lastName } = box(simpson);

return (
            <label>First name: <TextInput text={firstName}/></label>
            <label>Last name: <TextInput text={lastName}/></label>

This achieves simple two-way binding, via the box function, with obvious clarity and static type-safety. If we'd just said person.lastName we'd be passing the value, so the TextInput component would not be able to modify the value, but by saying box(person).lastName we're passing a wrapper that supports both get and set operations on the value of person.lastName.

Advanced API

You can create a box-like function from any function that can create a BoxValue, by wrapping it with the boxer higher order function:

function boxer(
    propertyBoxer: (obj: any, key: string) => BoxedValue<any>
): <T>(obj: T) => BoxedObject<T>;

So it accepts a "dynamic" means of boxing a single property, and returns a function that provides the same facility through the statically type-checked API of boxm.

The function makeBoxedValue, is suitable for passing directly to boxer to duplicate the standard behaviour:

function makeBoxedValue(obj: any, key: string): BoxedValue<any>;

(The returned object has a prototype, which serves as a hint to MobX to handle it transparently.)

Combining these two pieces, we get the implementation of box:

const box = boxer(makeBoxedValue);

For example, this is how bidi-mobx defines its own MobX-optimal version of box. It falls back to makeBoxedValue if it can't do any better:

const box = boxer((obj, key) => {
    const atom = (isObservable(obj, key) || isComputed(obj, key)) 
        && extras.getAtom(obj, key) as any as BoxedValue<any>;

    return atom || makeBoxedValue(obj, key);




Abstraction over the mutable properties of an object hierarchy







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