- Handle a form's submit event in React
- Use controlled inputs to validate values
In this lesson, we'll discuss how to handle forms submitting in React.
If you want to code along there is starter code in the src
folder. Make sure
to run npm install && npm start
to see the code in the browser.
Now that we've learned how to control a form with state, we want to set up a way
to submit our form. For this, we use a different type of event listener,
onSubmit
, added to the form
in JSX:
return (
<form onSubmit={handleSubmit}>
<input type="text" onChange={handleFirstNameChange} value={firstName} />
<input type="text" onChange={handleLastNameChange} value={lastName} />
<button type="submit">Submit</button>
</form>
);
Now, whenever the form is submitted (by pressing the Enter or Return key in an
input field, or clicking a Submit button), a callback function will be called,
handleSubmit
. We don't have a function handleSubmit
yet, so let's write one
out:
function handleSubmit(event) {
event.preventDefault();
const formData = {
firstName: firstName,
lastName: lastName,
};
props.sendFormDataSomewhere(formData);
setFirstName("");
setLastName("");
}
Let's look at each line of code in this function:
event.preventDefault()
: The default behavior of a form is to try and submit the form data based on a defined action, which effectively causes the browser to refresh the page. We didn't (and don't need to) define an action. The result, however, is that the form makes a new request to the current page, causing a refresh. By usingevent.preventDefault()
, we stop this behavior from happening.
const formData = { firstName: firstName, lastName: lastName }
: Here, we are putting together the current form data into an object using the values stored in state.props.sendFormDataSomewhere(formData)
: A form, when submitted should send the form data somewhere. As mentioned a moment ago, the traditional HTML way was to send data to a server or another page using theaction
attribute. In React, we handle requests with asynchronous JavaScript. We won't go into the details of how this works just yet, but we can think ofsendFormDataSomewhere()
as the code that handles sending our data off. This function might be defined in the same form component, or can be passed down as a prop.setFirstName("")
: if we want to clear the input fields, all we need to do is set state! In a traditional JavaScript form, you might do something likeevent.target.reset()
to clear out the form fields. Here, we need to ensure that our component state matches what the user sees in the form. By setting state to an empty string, we're keeping our React component state in sync with what the user sees on the page.
You can contrast this to handling an uncontrolled form being submitted, in which case you would need to access the input fields from the DOM instead of accessing the values from state:
function handleSubmit(event) {
event.preventDefault();
// in an uncontrolled form, you need to access the input fields from the DOM
const formData = {
firstName: e.target[0].value,
lastName: e.target[1].value,
};
props.sendFormDataSomewhere(formData);
}
We don't have a server to send our data to, but to demonstrate submission, we
could modify our Form
component to list out submissions, storing them in
state:
import React, { useState } from "react";
function Form() {
const [firstName, setFirstName] = useState("John");
const [lastName, setLastName] = useState("Henry");
const [submittedData, setSubmittedData] = useState([]);
function handleFirstNameChange(event) {
setFirstName(event.target.value);
}
function handleLastNameChange(event) {
setLastName(event.target.value);
}
function handleSubmit(event) {
event.preventDefault();
const formData = { firstName: firstName, lastName: lastName };
const dataArray = [...submittedData, formData];
setSubmittedData(dataArray);
setFirstName("");
setLastName("");
}
const listOfSubmissions = submittedData.map((data, index) => {
return (
<div key={index}>
{data.firstName} {data.lastName}
</div>
);
});
return (
<div>
<form onSubmit={handleSubmit}>
<input type="text" onChange={handleFirstNameChange} value={firstName} />
<input type="text" onChange={handleLastNameChange} value={lastName} />
<button type="submit">Submit</button>
</form>
<h3>Submissions</h3>
{listOfSubmissions}
</div>
);
}
export default Form;
The above component will render previous form submissions on the page! We have a fully functioning controlled form.
One benefit we get from having our form's input values held in state is an easy way to perform validations when the form is submitted. For example, let's say we want to require that a user enters some data into our form fields before they can submit the form successfully.
In our handle submit, we can add some validation logic to check if the form inputs have the required data, and hold some error messages in state:
// add state for holding error messages
const [errors, setErrors] = useState([]);
function handleSubmit(event) {
event.preventDefault();
// first name is required
if (firstName.length > 0) {
const formData = { firstName: firstName, lastName: lastName };
const dataArray = [...submittedData, formData];
setSubmittedData(dataArray);
setFirstName("");
setLastName("");
setErrors([]);
} else {
setErrors(["First name is required!"]);
}
}
Then, we can display an error message to our user in the JSX:
return (
<div>
<form onSubmit={handleSubmit}>
<input type="text" onChange={handleFirstNameChange} value={firstName} />
<input type="text" onChange={handleLastNameChange} value={lastName} />
<button type="submit">Submit</button>
</form>
{/* conditionally render error messages */}
{errors.length > 0
? errors.map((error, index) => (
<p key={index} style={{ color: "red" }}>
{error}
</p>
))
: null}
<h3>Submissions</h3>
{listOfSubmissions}
</div>
);
By setting up our form components using controlled inputs, we give React state control over the data being displayed in the DOM. As a benefit of having all the form data in state, we can more easily access form data via state when a form is submitted, so that we can send that data to another component, or use it to make a fetch request. We can also more easily perform some validation logic when the form data submitted.