Before we get started, it will benefit you greatly if you read my post on encapsulation first. In that post, I argue that the primary purpose of a function is to encapsulate all the elements of a concern together into a single structure. We're going to apply this pattern to how we work with React Hooks.
The Problem
Given the nature of our work, that requirements change frequently and therefore code changes frequently, too, it is not surprising that our codebases become messy. Our logic gets strewn about like tools left on a workbench. The problem is that hidden in the mess of functions and objects is essential information that needs to be recovered. Important historical decisions are left without explanation or context and more. I'll try to come up with a contrived example quickly.
Imagine a Component that can be toggled into two states: off and on. We'll use React Hooks to set this up.
function Component() {
const [state, setState] = React.useState('off')
const on = React.useCallback(() => {
setState('on')
}, [])
const off = React.useCallback(() => {
setState('off')
}, [])
const toggle = React.useCallback(() => {
setState(s => (s === 'on' ? 'off' : 'on'))
}, [])
return (
<div>
<div>State: {state}</div>
<div>
<button type="button" onClick={on}>
Turn On
</button>
<button type="button" onClick={off}>
Turn Off
</button>
<button type="button" onClick={toggle}>
Toggle
</button>
</div>
</div>
)
}
Alright, so far so good. Our Component is pretty simple, and all of its concerns are handled orderly. But requirements change, and perhaps our component also needs an input field (remember to ask me why I told you this is contrived).
function Component() {
const [toggleState, setToggleState] = React.useState('off')
const [inputState, setInputState] = React.useState('')
const on = React.useCallback(() => {
setToggleState('on')
}, [])
const off = React.useCallback(() => {
setToggleState('off')
}, [])
const toggle = React.useCallback(() => {
setToggleState(s => (s === 'on' ? 'off' : 'on'))
}, [])
const handleInputChange = React.useCallback(e => {
setInputState(e.target.value)
}, [])
const resetInput = React.useCallback(() => {
setInputState('')
}, [])
return (
<div>
<div>State: {toggleState}</div>
<div>
<button type="button" onClick={on}>
Turn On
</button>
<button type="button" onClick={off}>
Turn Off
</button>
<button type="button" onClick={toggle}>
Toggle
</button>
</div>
<div>
<label htmlFor="randomWord">Random Word</label>
<input
type="text"
id="randomWord"
onChange={handleInputChange}
value={inputState}
/>
<button type="button" onClick={resetInput}>
Reset Input
</button>
</div>
</div>
)
}
Are you starting to see the problem I'm seeing? Even though we wrote relatively clean and clear code, we have started to create a gap between related implementation details.
We need to call our hooks in the same order every render of Component. Those are the rules. To accomplish this, we've followed a common organizational pattern with our declarations of state near the top of our component and our various event handlers further down. But in following this pattern, we've separated the toggle state and its event handlers with the interruption of declaring another instance of useState. Even worse, our input state is separated from its related handlers by three unrelated function declarations. Just imagine this in your codebase and I'm sure you've seen far worse! This can quickly become a nightmare.
There is, fortunately, a very simple solution: custom hooks.
Why Custom Hooks Are the Solution
A custom hook is just a function and functions are structures we can use to encapsulate the related elements of a concern and expose an API to our function's consumer. In the case of our Component, these custom hooks are fairly simple to create.
function useOnOff() {
const [state, setState] = React.useState('off')
const handlers = React.useMemo(
() => ({
on: () => {
setState('on')
},
off: () => {
setState('off')
},
toggle: () => {
setState(s => (s === 'on' ? 'off' : 'on'))
},
}),
[]
)
return [state, handlers]
}
function useInput() {
const [state, setState] = React.useState('')
const handlers = React.useMemo(
() => ({
handleInputChange: e => {
setState(e.target.value)
},
resetInput: () => {
setState('')
},
}),
[]
)
return [state, handlers]
}
function Component() {
const [toggleState, { on, off, toggle }] = useOnOff()
const [inputState, { handleInputChange, resetInput }] = useInput()
return (
<div>
<div>State: {toggleState}</div>
<div>
<button type="button" onClick={on}>
Turn On
</button>
<button type="button" onClick={off}>
Turn Off
</button>
<button type="button" onClick={toggle}>
Toggle
</button>
</div>
<div>
<label htmlFor="randomWord">Random Word</label>
<input
type="text"
id="randomWord"
onChange={handleInputChange}
value={inputState}
/>
<button type="button" onClick={resetInput}>
Reset Input
</button>
</div>
</div>
)
}
Take notice, our Component now only consumes custom hooks. These custom hooks give us a little more context to what they mean, and we don't have the implementation details of our state handler functions sitting in the middle of our component function.
Another benefit of this pattern is that dependencies quickly become obvious because they end up being arguments to our custom hooks. What if our useInput hook should begin with an initial state other than an empty string and we use that for resetting the input as well?
function useInput(initialState = '') {
const [state, setState] = React.useState(initialState)
const handlers = React.useMemo(
() => ({
handleInputChange: e => {
setState(e.target.value)
},
resetInput: () => {
setState(initialState)
},
}),
[initialState]
)
return [state, handlers]
}
function Component({ startingWord }) {
//...
const [inputState, { handleInputChange, resetInput }] = useInput(startingWord)
//...
}
There are even more benefits. By adding this layer of abstraction between our component and the standard React hooks, we can change the implementation of our custom hook's API without changing the component. Perhaps I want to use a reducer instead of useState for our useOnOff hook:
function onOffReducer(state, action) {
switch (action) {
case 'ON':
return 'on'
case 'OFF':
return 'off'
case 'TOGGLE':
return state === 'on' ? 'off' : 'on'
default:
return state
}
}
function useOnOff() {
const [state, dispatch] = React.useReducer(onOffReducer, 'off')
const handlers = React.useMemo(
() => ({
on: () => {
dispatch('ON')
},
off: () => {
dispatch('OFF')
},
toggle: () => {
dispatch('TOGGLE')
},
}),
[]
)
return [state, handlers]
}
Obviously, this type of change might be completely unnecessary for my example, but I hope you can recognize where this layer of abstraction might be useful to you when you have to change the implementation details of a hook. We were able to replace the entire state management of our hook without changing the API, which means our component behaves the same way, even though the implementation of that behavior has changed quite a bit.
How to Enforce This Pattern
What good is a pattern if you can't find a way to get others to use it? To steer them down the good path? We can do that for this pattern with an ESLint plugin and rule. Thus, I give you the eslint-plugin-use-encapsulation and the prefer-custom-hooks rule!
Following the installation instructions, you'll see that prefer-custom-hooks warn you whenever you use a React hook directly inside a component. The only way around the warning (other than disabling the rule), is to use the React hooks from within a custom hook.
Summary
By opting to write all the hooks consumed by your components as custom ones, you will provide future devs (including yourself) practical context by encapsulating all the pieces of concern into a single function. By doing this, you gain all the benefits of proper encapsulation and make your components more declarative. You might even gain a few useful reusable hooks in the process.
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