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Type-checking React useReducer in TypeScript

How TypeScript discriminating unions provide type safety to the useReducer hook

November 22, 2020 · 6 min read

The biggest benefit I’ve found with TypeScript is type-checking functions. TypeScript ensures that I pass the correct types to a helper function and it provides the type of the return value for me to use. Similarly, when writing functions I can be assured that I’m receiving the correct types as arguments and that I return the type that I intend.

In React, this manifests itself in several ways. The main way is type-checking props. After all, React components are just special functions with input (props) and output (markup). And we can do all kids of sophisticated things leveraging TypeScript’s ability to type-check functions, like setting up conditional React props, creating polymorphic React components, and defining generic React components.

React also has hooks which are special types of functions that let us use state and other React features using function components. In general, there’s nothing really special needed to type hooks. For instance, we can let type inference do all the work with useState:

import React, { useState } from 'react'

const Example = () => {
  // with type inference TypeScript knows that
  // `count` is a number` & `setCount` takes a number
  // based on `0` being passed as an initial value
  const [count, setCount] = useState(0)

  return (
    <div>
      <p>You clicked {count} times!</p>
      <button onClick={() => setCount((curCount) => count + 1)}>
        Click me
      </button>
    </div>
  )
}

Because of how useState is typed (it’s a generic function), count is inferred to be a number and setCount is inferred to be a function that is passed a number. This is all because of the initial value 0 (a number) that was passed to useState.

However, useReducer is one hook that requires additional type hand-holding. The docs suggest using useReducer over useState when we “have complex state logic that involves multiple sub-values.” But to try to keep things as simple as possible, let’s take the traditional Redux todos example and convert it to useReducer + Typescript:

type Visibility = 'all' | 'completed' | 'active'
interface State {
  todos: {
    id: number
    text: string
    completed: boolean
  }[]
  visibility: Visibility
}
type Action =
  | { type: 'add_todo'; id: number; text: string }
  | { type: 'toggle_todo'; id: number }
  | { type: 'set_visibility'; visibility: Visibility }

const reducer = (state: State, action: Action): State => {
  switch (action.type) {
    case 'add_todo':
      return {
        ...state,
        todos: [
          ...state.todos,
          { id: action.id, text: action.text, completed: false },
        ],
      }
    case 'toggle_todo':
      return {
        ...state,
        todos: state.todos.map((todo) =>
          todo.id === action.id
            ? { ...todo, completed: !todo.completed }
            : todo,
        ),
      }
    case 'set_visibility':
      return {
        ...state,
        visibility: action.visibility,
      }
  }
}

let nextTodoId = 0
const Example = () => {
  const [state, dispatch] = useReducer(reducer, {
    todos: [],
    visibility: 'all',
  })

  return (
    <main>
      <h1>Todos!</h1>

      <TodoForm
        onSubmit={(text) => {
          dispatch({ type: 'add_todo', text, id: nextTodoId++ })
        }}
      />

      <TodosList
        todos={state.todos}
        filter={state.visibility}
        onToggleCompleted={(id) => {
          dispatch({ type: 'toggle_todo', id })
        }}
      />

      <RadioGroup
        value={state.visibility}
        onChange={(newVisibility) =>
          dispatch({ type: 'set_visibility', visibility: newVisibility })
        }
      >
        <Radio value="all">All</Radio>
        <Radio value="completed">Completed</Radio>
        <Radio value="active">Active</Radio>
      </RadioGroup>
    </main>
  )
}

Ok, I know that may look like a lot. But it’s actually only the top portion that is the TypeScript addition. The majority of the code is the implementation which leverages type inference to ensure type safety.

Let’s break thing down bit by bit.

type Visibility = 'all' | 'completed' | 'active'
interface State {
  todos: {
    id: number
    text: string
    completed: boolean
  }[]
  visibility: Visibility
}

First we define the shape of our state. Just like with props, we can’t get away with hand-waving the state shape or defining it implicitly in code. It has to be explicitly defined. In this case, our state has two required properties: todos (an array of todo objects with their own required properties) and visibility (an “enum” of visibility states).

type Action =
  | { type: 'add_todo'; id: number; text: string }
  | { type: 'toggle_todo'; id: number }
  | { type: 'set_visibility'; visibility: Visibility }

Next, using a discriminated union, we define the valid actions. This discriminated union is what becomes the linchpin and provides the strong typing for useReducer.

It means that an action can only have one of these 3 shapes. The fact that we cannot have a type other than the 3 listed catches any typos and prevents the need for having shared constants. An 'add_todo' action must have both id & text properties, whereas a 'toggle_todo' cannot have a text property. The visibility property for 'set_visibility' must be one of the 3 Visibility options (which also matches the visibility state property).

const reducer = (state: State, action: Action): State => {
  switch (action.type) {
    case 'add_todo':
      return {
        ...state,
        todos: [
          ...state.todos,
          { id: action.id, text: action.text, completed: false },
        ],
      }
    case 'toggle_todo':
    // ...
    case 'set_visibility':
    // ...
  }
}

The reducer function is specifically typed to take State and Action arguments, and return a State object. This function definition provides enough type information that the rest of the reducer can be strongly typed just using type inference.

When we use the switch on action.type, we can only have case statements for 'add_todo', 'toggle_todo' and 'set_visibility'. Any other case values will be an error. Furthermore, if we leave off one of the actions from the switch, TypeScript will know that there’s a chance that the function can return undefined. And because we say the reducer must return State, TypeScript will complain thanks to union exhaustiveness checking.

Within a given case statement, TypeScript now knows which action we are in. Therefore trying to access action.visibility within 'add_todo', action.text within 'toggle_todo' or action.id within 'set_visibility' will all be errors. Once again this is all thanks to the Action being a discriminated union.

Last, but not least, the return values must be valid State objects. So if I forget to ...state, I will be missing properties of State and TypeScript will complain.

All pretty sweet, huh? 🤯

let nextTodoId = 0
const Example = () => {
  const [state, dispatch] = useReducer(reducer, {    todos: [],    visibility: 'all',  })
  return (
    <main>
      <h1>Todos!</h1>
      <TodoForm
        onSubmit={(text) => {
          dispatch({ type: 'add_todo', text, id: nextTodoId++ })        }}
      />
      ...
    </main>
  )
}

Now, down in the component itself, useReducer (also a generic function by the way) is able to infer types based on the type of the reducer function. So it knows state is State and that dispatch is a function that can only dispatch Action objects. In addition, TypeScript ensures that the initial state (the second parameter of useReducer) is a full State object.

Finally, when we call dispatch, it must be a valid Action object. There’s no need to use a constant for the 'add_todo' type because it must match one of the valid Action types. If we were to change the name of the type to just 'add', we would get TypeScript errors.


So that’s it! Properly typing useReducer has a couple of extra moving parts in TypeScript. You have to define your state and your actions. But we’re already implicitly defining it in our code, so I find that it’s nice to have it be explicit. If useReducer is intended to be used when your state is complex, it’s probably best that everything is defined up front.

And then of course we gain tons of type safety that will help us sleep better at night. 😄

Keep learning my friends. 🤓

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Hi, I'm Ben Ilegbodu. 👋🏾

I'm a Christian, husband, and father of 3, with 15+ years of professional experience developing user interfaces for the Web. I'm a Principal Frontend Engineer at Stitch Fix, frontend development teacher, Google Developer Expert, and Microsoft MVP. I love helping developers level up their frontend skills.

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