How to Reduce Action Boilerplate

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I use Redux for my application development and, to take advantage of RxJS, I use NgRx in Angular projects and redux-observable in React projects. I also use TypeScript.

Unfortunately, the amount of boilerplate required for TypeScript to be effective with Redux can be disheartening. In his article Introducing @ngrx/entity, Mike Ryan shows how @ngrx/entity can be used to write CRUD reducers with little code. It’s great. And much appreciated. However, it doesn’t help with the TypeScript cruft in action declarations.

In the past, I’ve resorted to code generation — using doT — to avoid the usual repetition. More recently, I’ve investigated alternative approaches and I’ve found one that’s terse and suits my needs.

Before I introduce the library I’ve written, let’s look at how TypeScript works with Redux.

Redux is fundamentally about the dispatch and receipt of actions, and TypeScript has benefits for both.

When dispatching an action, the use of action creators — rather than object literals — is recommended. There are a number of reasons for using action creators — including brevity, encapsulation and testability — but TypeScript offers another: type safety. Strongly typed actions will prevent the omission of required properties, the inclusion of unnecessary properties, and the inclusion of properties that have the incorrect type. However, there’s nothing complicated here: you just create an action using a TypeScript class or method and pass it to dispatch. It’s where actions are received that problems arise.

In Redux, actions are simple, anonymous objects, so when an action is received, its type property is all that there is to work with. (If you create an action using a class, there is no guarantee that when it’s received it will still be an instance of that class — it could be an action replayed by the Redux DevTools — so instanceof cannot be used.)

Typically, the base Redux action will be defined using an interface and will look like this:

When a reducer receives an action like this:

We want to work with it not as an Action, but as a type that also includes the text and id properties. In TypeScript, this is referred to as type narrowing and there are two mechanisms for performing narrowing: type guards; and discriminated unions.

TypeScript supports typeof and instanceof type guards, but for Redux actions, a user-defined type guard is what’s required. A user-defined type guard is a function that performs a run-time check to evaluate its returned type predicate.

For example, if we have this interface:

We can write a user-defined type guard to determine whether an Action is an AddTodo action. The user-defined type guard looks like this:

Of particular interest is the function signature’s return type: action is AddTodo. This is the type predicate and it’s what makes the function a type guard.

We can use our type guard to write a reducer like this (the example reducers in this article perform some basic CRUD actions by manipulating arrays; if you are using NgRx, I’d recommend using @ngrx/entity instead):

TypeScript will recognise the use of the type guard and, inside the if statement, it will be aware that theaction instance is of the type AddTodo and will have text and id properties.

Discriminated unions are best explained by example, so lets create one using these interfaces:

Both are Redux actions, so they each have type properties, but it’s the types of those properties that are important. The type properties in the interfaces are not declared as string; instead, the are declared using distinct, string-literal types: "ADD_TODO"; and "REMOVE_TODO".

A property with a string-literal type is not just a string; it’s a string that can only have the specified value. So the type property in AddTodo can only have a value of "ADD_TODO". TypeScript is able to narrow a union of types in which all of the types share a common, string-literal property.

With the interfaces, we can write a reducer like this:

Note the type of the reducer function’s action parameter: AddTodo | RemoveTodo. It’s a union type and it tells TypeScript that the action parameter will be either AddTodo or RemoveTodo. With that information — and the with the common, distinct, string-literal type properties in the interfaces — TypeScript will narrow the type of action within the case statements.

Now that we’ve looked at the two narrowing mechanisms, let’s look at how they are used in two different implementations: NgRx and typescript-fsa.

The approach NgRx takes — as illustrated Mike’s article — uses a discriminated union for the narrowing. Classes are used as action creators and their declarations look like this:

The constants associated with the actions’ type properties are declared in an enum and the enum is exported, so that its members can be used in reducers.

Actions are declared as classes and the enum constants are assigned to the classes’ type properties. Because the type properties are readonly, TypeScript will infer a string-literal type for the property. This is a key point. If the type properties are not declared as readonly, TypeScript will widen the inferred type to string — as the properties could be re-assigned a string with an arbitrary value.

A union type — of all the action classes — is also exported for use in the reducer and the reducer looks something like this:

typescript-fsa is a small action-creator library for Flux-standard actions — which have this shape:

It takes a different approach to NgRx and uses user-defined type guards to perform the narrowing. Instead of using classes as action creators, typescript-fsa uses functions that accept anonymous objects that have a specified shape. And those action creator functions are created using a factory, like this:

And, in the reducer, the narrowing is performed with the isType function — which is a user-defined type guard:

The approach taken by typescript-fsa involves less boilerplate than that taken by NgRx. However, there were a number of reasons why I was reluctant to adopt typescript-fsa:

  • the actions I’d been using didn’t have the shape of Flux-standard actions — it’s common in Redux to add properties at the same level as the type property, rather than under a payload property; and
  • all of my reducers used switch statements.

I just wanted to eliminate the code-generation in my projects, as it complicated the build process. I didn’t want to have to change the action structure or re-write the reducers.

Instead, I wrote a library — ts-action — that takes a different approach and can narrow using either type guards or discriminated unions.

Like NgRx, ts-action uses classes as action creators. However, those classes are created through calls to its action method, so the action creator declarations look like this:

The props method will place the specified properties at the same level as the type property. To place them inside a payload property, the payload method can be used instead.

ts-action also includes base method that allows for the base class to be specified inline, like this:

Inline base classes offer flexibility for property defaults and initialization, etc.

The classes created by ts-action explicitly reset each action’s prototype, so that they are compatible with reactjs/redux — that is, so that each action is considered to be a plain object.

The action creators can be used in reducers that narrow using a discriminated union, like this:

And they can be used in reducers that narrow using user-defined type guards, like this:

Being RxJS-based, NgRx also includes some methods that act like operators and can be used to filter and narrow an observable of actions. In particular, the Actions observable used with @ngrx/effects has an ofType property and it looks like this:

I wanted an operator that could be used with the action creators in ts-action, so I created ts-action-operators. It contains an ofType operator that accepts an action creator:

With all of the information in the action creator, there is no need to specify a type parameter, as the ofType operator is implemented using the isType user-defined type guard in ts-action.

ts-action-operators is independent of NgRx, so it can be used with redux-observable epics, too.

Overall, I’m pleased with ts-action. With it, I've been able to remove a reasonable amount of boilerplate, and, building it, I’ve learned rather a lot about some of TypeScript’s less-often-used features. And what I’ve learned will likely be the subject of my next article.

This post is also published on my personal blog:

RxJS core team member; front-end developer; mentor; speaker; open-source contributor

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