React offers 2 approaches to access the value of an input field: using a controlled or uncontrolled inputs techniques. I prefer the controlled because you read and set the input value through the component's state.
In this post, you'll read how to implement controlled inputs using React hooks.
Table of Contents
1. The controlled input
Let's say you have a simple text input field, and you'd like to access its value:
import { useState } from 'react';function MyControlledInput() { const [value, setValue] = useState(''); const onChange = (event) => { setValue(event.target.value); }; return ( <> <div>Input value: {value}</div> <input value={value} onChange={onChange} /> </> );}
Open the demo and type into the input field. value state variable contains the value entered into the input field that is updated each time you type into the input field.
The input field is controlled because React sets its value from the state <input value={value} ... />. When the user types into the input field, the onChange handler updates the state with the input’s value accessed from the event object: event.target.value.
value state variable is the source of truth. Each time you need to access the input value — just read value state variable.
To resume, the controlled input technique requires 3 steps:
- Define the state to hold the input value:
const [value, setValue] = useState('')
- Create the event handler that updates the state when the input value changes:
const onChange = event => setValue(event.target.value)
- Assign the input field with the state value and attach the event handler:
<input type="text" value={value} onChange={onChange} />
The controlled components approach can help you access the value of any input type: textual inputs, textareas, select fields.
In case of a checkbox, however, you have to use checked prop instead of value:
<input checked={value} onChange={onChange} type="checkbox" />
2. Controlling multiple inputs
Often you have to deal with forms that contain multiple input fields. In such a case, instead of creating many state variables for each input field, I find it useful to use a single object to keep the state of the input fields.
Each input field has a corresponding property in the state object.
For example, let's use an object values having the properties first and last to hold the information of first and last name input fields.
import { useState } from 'react';export default function MyControlledInputs() { const [values, setValues] = useState({ first: '', last: '' }); const getHandler = (name) => { return (event) => { setValues({ ...values, [name]: event.target.value }); }; }; return ( <> <div>Name: {values.first} {values.last}</div> <input value={values.first} onChange={getHandler('first')} /> <input value={values.last} onChange={getHandler('last')} /> </> );}
The state of the component is now an object values. That's usually shorter than creating state variables for each input field.
getHandler is a factory function that returns event handlers to update the property supplied as an argument. The returned event handler is assigned to onChange prop of the input field.
The benefit of such a design is that you can handle a lot of input fields without adding too much boilerplate code.
3. The state as the source of truth
Let's see a more complex example. A web page consists of a list of employees' names. You need to add an input field, and when the user types into this field, the employees' list is filtered by name.
That's a good scenario to use a controlled input. Here's a possible implementation:
function FilteredEmployeesList({ employees }) { const [query, setQuery] = useState(''); const onChange = event => setQuery(event.target.value); const filteredEmployees = employees.filter(name => { return name.toLowerCase().includes(query.toLowerCase()); }); return ( <div> <h2>Employees List</h2> <input type="text" value={query} onChange={onChange} /> <div className="list"> {filteredEmployees.map(name => <div>{name}</div>)} </div> </div> );}
Open the demo and enter a query into the input field. The list of employees is filtered.
What's important is that query state variable is the source of truth for the value entered in the input field. You use it inside employees.filter() to filter the list of employees: name.toLowerCase().includes(query).
4. Debouncing the controlled input
In the previous implementation, as soon as you type a character into the input field, the list gets filtered instantly. That's not always convenient because it distracts the user when typing the query.
Let's improve the user experience with debouncing: filter the list with a delay of 400 ms after the last input change.
Let's see a possible implementation of a debounced controlled input:
import { useDebouncedValue } from './useDebouncedValue';function FilteredEmployeesList({ employees }) { const [query, setQuery] = useState(''); const debouncedQuery = useDebouncedValue(query, 400); const onChange = event => setQuery(event.target.value); const filteredEmployees = employees.filter(name => { return name.toLowerCase().includes(debouncedQuery.toLowerCase()); }); return ( <div> <h2>Employees List</h2> <input type="text" value={query} onChange={onChange} /> <div className="list"> {filteredEmployees.map(name => <div>{name}</div>)} </div> </div> );}
Open the demo and enter a query into the input field. The employees' list doesn't filter while you type, but after passing 400ms after the latest keypress.
The new state value debouncedQuery value is managed by a specialized hook that implements debouncing: useDebouncedValue(query, 400) . debouncedQuery state value is used to filter the employees' list and is derived from the input value state.
Here's the implementation of useDebouncedValue():
export function useDebouncedValue(value, wait) { const [debouncedValue, setDebouncedValue] = useState(value); useEffect(() => { const id = setTimeout(() => setDebouncedValue(value), wait); return () => clearTimeout(id); }, [value]); return debouncedValue;}
In a few words, here's how it works.
First, the useDebouncedValue() hook creates a new state derived from the main state.
Then, useEffect() updates after wait delay the debouncedValue state when the main value state changes.
5. Summary
The controlled input is a convenient technique to access values of input fields in React.
Setting up a controlled input requires 3 steps:
- Create the state to hold the input value:
const [val, setVal] = useState('')
- Define the event handler to update the state when the user types into the input:
onChange = event => setVal(event.target.value) - Attach the event handler and set
valueattribute on the input field:<input onChange={onChange} value={val} />.
Debouncing of the input value state requires creating a new derived state using the specialized hook debouncedQuery = useDebouncedValue(value, wait).
Do you have any question regrading controlled inputs?
