Daniel Earwicker Chief Software Architect
FISCAL Technologies Ltd

Box 'em! - Property references for TypeScript


This concerns quite an abstract, simple building block, but it is a neat tool for use with React and MobX. In MobX there's a utility observable.box (docs). But I don't want to use that create all my properties and have to put .get() after every read access. I want to use the cool @observable decorator and just fetch my properties directly, and assign new values with =. What I need is a way to box a property. Oh, and it better be statically type checked in TypeScript.

For the overall idea, see the project page, or just look at the takeaway:

// MobX model:
class Person {
    @observable firstName: string;
    @observable lastName: string;
    @observable dateOfBirth: Date;

// Two-way binding in React component:
const { firstName, lastName, dateOfBirth } = box(props.person);
return (
        <label>First name: <TextInput value={firstName} /></label>
        <label>Last name: <TextInput value={lastName} /></label>
        <label>Date of birth: <DateInput value={dateOfBirth} /></label>

That is, incredibly easy, tidy, declarative and statically-typed two-way binding. I'm working on another project called bidi-mobx that will exploit this idea and provide examples (including validation).

Here I want to mention how it uses a few new-ish TypeScript features, and where it calls for possible future extensions to the language.

Way back when Roslyn first went open source and I got excited about how hackable it was, I added A new kind of managed lvalue pointer. Just like you can get a reference to a method (called a delegate in C# - and, wonder of wonders, it's always properly bound to the right this), why not a reference to a specific mutable property? It's a bundling together of two methods: a getter and a setter. The pain is having no neat syntax to construct the reference, specifying the object and the property just once each.

And as if in a demonstration of how TypeScript is way better than C#, you can now achieve the same end without having to change the compiler! It's statically type checked, although there is a limitation to this (and I'm not entirely sure how it would be fixed, but I wouldn't bet against TS fixing it soon).

It's thanks to three separate features that play together very nicely. First, keyof T is a union of string literal types, in which each string is the name of a property of T. So it's a compile-time analogue of Object.keys.

Second, if K is a string literal type and T has a property of that name, the type expression T[K] gives us the type of that property. This is a type indexer. The syntax looks just like the runtime JavaScript indexing operation where you pass a string property name to get its value. One way to think of this is that an interface is a collection (a dictionary or map) of types, keyed by names, and now we have a way to fetch an item from the collection. Maybe next we'll have conditional types, and then who knows where we might end up. Conditions need booleans to steer them, but TypeScript already has true and false as types. Hold onto your hats…

One slight wrinkle is that T[K] identifies a property, which might have the readonly modifier. Type indexing seems to ignore that modifier so that information is lost. This leads to the type hole I mentioned above. We want to target mutable properties only, but we can't tell if a property is readonly (or do much about it, even if we could tell - this is one place where conditional types would come in handy).

Finally, there are now mapped types, which is a way of declaring a set of properties with a single declaration, e.g. there's now a standard definition in the TS core library:

type Readonly<T> = {
    readonly [P in keyof T]: T[P];

The magic part is the [P in keyof T] which roughly means "repeat this line for each property of T", so if T has five properties then so will Readonly<T>. This is like Array#map, but operating on types at compile time instead of values at runtime. It might grow into a compile-time form of list comprehension.

My use of these features is structurally very similar to Readonly<T>, except I wrap each property in another type:

export type BoxedObject<T> = {
    readonly [K in keyof T]: BoxedValue<T[K]>;

And that type is:

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

So a BoxedObject<T> has properties that have the same name as the properties of T, and each property type P is "amplified" (wrapped) to become BoxedValue<P>. By the way, BoxedValue has been designed to match the shape of those boxed objects available at a low level in MobX. So if you define React components to accept BoxedValue props, they will be broadly compatible.

The only other thing is to look at the implementation. Actually there are two. Implementing BoxedValue is trivial, but BoxedObject must seem to have a property when you ask it for one. The fast way to do this (I've timed it) is to use Proxy.

const handler: ProxyHandler<any> = {
    get(target: any, key: PropertyKey) {
        return makeBoxedValue(target, key);

How neat is that? Unfortunately no version of IE has Proxy, so there's a fallback that just generates an object that has every property of the underlying object created on it.

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