React Forms Submit

Learning Goals

• Handle a form's submit event in React
• Use controlled inputs to validate values

Introduction

In this lesson, we'll discuss how to handle form submission 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.

Submitting a Controlled Form

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 add the onSubmit event listener to our form element:

// src/components/Form.js
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 by clicking a Submit button), the handleSubmit callback function will be called. We don't have the handleSubmit function yet, so let's write it out:

// src/components/Form.js
function handleSubmit(event) {
  event.preventDefault();
  const formData = {
    firstName: firstName,
    lastName: lastName,
  };
  props.sendFormDataSomewhere(formData);
  setFirstName("");
  setLastName("");
}

We're missing something though - the typing for the event parameter. TypeScript rightfully doesn't like that it defaults to the any type, so let's change that. We can use the IDE trick we learned to figure out what umbrella type the onSubmit event falls under. Hover over the onSubmit attribute in the JSX, and we should find that it's a React.FormEvent on an HTMLFormElement.

Let's type the parameter like so:

// src/components/Form.js
function handleSubmit(event: React.FormEvent<HTMLFormElement>) {
  // ...
}

Now, we can 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 using event.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 the action 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 of sendFormDataSomewhere() 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, as shown in our example.
• setFirstName(""): if we want to clear the input fields after the user submits something, all we need to do is set state! In a traditional JavaScript or TypeScript form, you might do something like event.target.reset() to clear out the form fields. Here, because we are using controlled inputs, setting state to an empty string clears out the values from the input fields once the data has been submitted.

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:

// src/components/Form.js
function handleSubmit(event: React.FormEvent<HTMLFormElement>) {
  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);
}

Since we don't have a server to send our data to, let's remove our sendFormDataSomewhere() function, the Form component's props parameter, and the Props interface. Instead, we'll demonstrate submission by modifying our Form component to access submitted values from state and list them in the DOM:

// src/components/Form.js
import { useState } from "react";

function Form() { /* code change, remove props */
  const [firstName, setFirstName] = useState("Beatriz");
  const [lastName, setLastName] = useState("Solórzano");
  // code change - create new state variable
  const [submittedData, setSubmittedData] = useState<NameFormData[]>([]);

  function handleFirstNameChange(event: React.ChangeEvent<HTLMInputElement>) {
    setFirstName(event.target.value);
  }

  function handleLastNameChange(event: React.ChangeEvent<HTMLInputElement>) {
    setLastName(event.target.value);
  }

  // code change - updated function to utilize our state variables
  function handleSubmit(event: React.FormEvent<HTMLFormElement>) {
    event.preventDefault();
    const formData = { firstName: firstName, lastName: lastName };
    const dataArray = [...submittedData, formData];
    setSubmittedData(dataArray);
    setFirstName("");
    setLastName("");
  }

  // code change - created new variable that maps out the data held in our submittedData state
  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>
      {/* code change - displays the mapped data */}
      {listOfSubmissions}
    </div>
  );
}

export default Form;

Note: You may also have to remove the prop being passed down to Form in the parent App component to appease TypeScript.

The above component will render previous form submissions on the page! We have a fully functioning controlled form.

Before we move on, let's address the following line:

const [submittedData, setSubmittedData] = useState<NameFormData[]>([]);

Here, we had to explicitly type our state because when initializing state with an empty array in React, type inference assumes the array has a type called never.

To avoid that, we explicitly type our submittedData as an array made up of NameFormData typed objects. You may have noticed that type in the original starter code as well - where did that come from? If we look at the top of the file, we'll see it's being imported from a types.ts file. Opening that file will reveal the NameFormData interface.

In this case, we defined the interface in a separate file because it's also being used in App.tsx. To avoid redundant definitions of the same exact interface, we instead opted to define it in a singular file that gets imported where needed. You may find yourself having to do so when your projects get bigger and data gets passed around.

Validating Inputs

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 enter some data into our form fields before they can submit the form successfully.

In our handleSubmit function, 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
// we MUST explicitly type our state array here since we're initializing it as an empty array
const [errors, setErrors] = useState<string[]>([]);

function handleSubmit(event: React.FormEvent<HTMLFormElement>) {
  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>
);

Note: Alternatively, there are some validation attributes built into HTML forms themselves, such as a required attribute, that can prevent users from submitting altogether if a specific input is not filled in correctly. Still, it's good to know how to do these validations by hand in more complex cases, such as checking that a password meets certain requirements.

Conclusion

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 the form data in state, we can more easily access it once a form is submitted and either pass it along to another component or use it to make a fetch request. We can also more easily perform some validation logic when the form data is submitted.

Resources

• React Forms
• Using built-in Form Validation