Hooks are a brilliant addition to React. They simplify a lot of logic that had to be earlier split up into different lifecycles with class components. They do, however, require a different mental model, especially for timers.
I also recorded a short video series on this article which you may find more explanatory.
You can also read this post on my personal blog which has a cleaner UI and better syntax highlighting.
Debounce and throttle
There are a ton of blog posts around debounce and throttle so I won't be diving into how to write our own debounce and throttle. For brevity, consider debounce and throttle from Lodash.
If you need a quick refresher, both accept a (callback) function and a delay in milliseconds (say x) and return another function with some special behavior:
-
debounce: returns a function that can be called any number of times (possibly in quick successions) but would only invoke the callback after waiting forxms from the last call. -
throttle: returns a function that can be called any number of times (possibly in quick successions) but would only invoke the callback atmost once everyxms.
Usecase
We've a minimal blog editor (GitHub repo) and we would like to save the blog post to the database 1 second after user stops typing.
If you would like to take a look at the final version, you can check out this Codesandbox.
A minimal version of our blog editor looks like:
import React, { useState } from 'react';
import debounce from 'lodash.debounce';
function App() {
const [value, setValue] = useState('');
const [dbValue, saveToDb] = useState(''); // would be an API call normally
const handleChange = event => {
setValue(event.target.value);
};
return (
<main>
<h1>Blog</h1>
<textarea value={value} onChange={handleChange} rows={5} cols={50} />
<section className="panels">
<div>
<h2>Editor (Client)</h2>
{value}
</div>
<div>
<h2>Saved (DB)</h2>
{dbValue}
</div>
</section>
</main>
);
}
Here, saveToDb would actually be an API call to the backend. For keeping things simple, I'm saving it in state and then rendering as dbValue. Since we only want to perform this save operation once user has stopped typing (after 1 second), this should be debounced.
Here's the starter code repo and branch.
Creating a debounced function
First of all, we need a debounced function that wraps the call to saveToDb:
import React, { useState } from 'react';
import debounce from 'lodash.debounce';
function App() {
const [value, setValue] = useState('');
const [dbValue, saveToDb] = useState(''); // would be an API call normally
const handleChange = event => {
const { value: nextValue } = event.target;
setValue(nextValue);
const debouncedSave = debounce(() => saveToDb(nextValue), 1000);
debouncedSave();
};
return <main>{/* Same as before */}</main>;
}
But, this doesn't actually work because the function debouncedSave is created fresh on each handleChange call. This would end up debouncing each keystroke rather than debouncing the entire input value.
useCallback
Although useCallback is commonly used for performance optimizations when passing callbacks to child components, we can use its constraint of memoizing a callback function to ensure the debouncedSave references the same debounced function across renders.
This works as expected:
import React, { useState, useCallback } from 'react';
import debounce from 'lodash.debounce';
function App() {
const [value, setValue] = useState('');
const [dbValue, saveToDb] = useState(''); // would be an API call normally
const debouncedSave = useCallback(
debounce(nextValue => saveToDb(nextValue), 1000),
[], // will be created only once initially
);
const handleChange = event => {
const { value: nextValue } = event.target;
setValue(nextValue);
// Even though handleChange is created on each render and executed
// it references the same debouncedSave that was created initially
debouncedSave(nextValue);
};
return <main>{/* Same as before */}</main>;
}
useRef
useRef gives us a mutable object whose current property refers to the passed initial value. If we don't change it manually, the value would persist for the entire lifetime of the component. This is similar to class instance properties (i.e. defining methods and properties on this).
This also works as expected:
import React, { useState, useRef } from 'react';
import debounce from 'lodash.debounce';
function App() {
const [value, setValue] = useState('');
const [dbValue, saveToDb] = useState(''); // would be an API call normally
// This remains same across renders
const debouncedSave = useRef(debounce(nextValue => saveToDb(nextValue), 1000))
.current;
const handleChange = event => {
const { value: nextValue } = event.target;
setValue(nextValue);
// Even though handleChange is created on each render and executed
// it references the same debouncedSave that was created initially
debouncedSave(nextValue);
};
return <main>{/* Same as before */}</main>;
}
Continue reading on my blog for how to extract these into custom hooks or check out the video series.
You may also follow me on Twitter for updates on related content.
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