Jordan Finneran

Posted on Dec 21, 2020 • Edited on Mar 13, 2021 • Originally published at jordanfinners.dev

How to easily convert HTML Form to JSON

#webdev #javascript #html #form

Intro
Form Data
Gotcha's
Examples
Summary

Intro

I have a love, but sometimes hate, relationship with HTML Forms. HTML Forms are absolutely brilliant for out of the box validation, accessibility and usability. But they can be a pain to style!
Nevertheless, HTML Forms give you massive amounts of functionality with zero dependencies.

If you are anything like me, you prefer your API to be JSON based. Making it easier and more consistent to deal with requests and responses on the backend.
You could add a middleware to your API endpoint that's going to handle your Form UI but why make that endpoint different from the rest?

What if you could send your Form data off in JSON format and handle it like all your other endpoints. Now you can!

TLDR; Skip to the examples

Form Data

Introducing FormData, this is a really nice Web API for manipulating data in HTML Forms.

This allows us to easily convert a HTML Form to JSON using the following.

Grab the form element from the DOM.

const formElement = document.querySelector('form')

Pass the form to the following function

/**
 * Creates a json object including fields in the form
 *
 * @param {HTMLElement} form The form element to convert
 * @return {Object} The form data
 */
const getFormJSON = (form) => {
  const data = new FormData(form);
  return Array.from(data.keys()).reduce((result, key) => {
    result[key] = data.get(key);
    return result;
  }, {});
};

Action the JSON Object result, however you want to handle the data from the form! Send it off with Fetch for example.

Gotcha's

As with everything these are a few gotchas to look out for!

Checkbox's

If ticked your checkbox's will appear in the result as 'on'. Probably not the boolean you want.
For example, if you've got this input in your form, which the user has checked.

<input name="isOpen" type="checkbox" checked />

It would produce, using the function above:

{
  "isOpen": "on"
}

You will probably want to check for the property and if it equals 'on' and convert it to a boolean.

File Uploads

This one really caught me out, if you've got a file input which accepts multiple files, like so:

<input name="uploads" type="file" multiple />

If one file is uploaded you will get a File object.
But if you have multiple files uploaded, you will actually get a list of them.

Fortunately, there is a really simple fix for this to consistently provide you with a list of files.
Grab the files from the result and process them like so:

[files].flat().filter((file) => !!file.name)

This will consistently give you a list of files, handling if only a single file is uploaded, no file is uploaded or multiples are uploaded.

This also means you can do more client side checks on file sizes and limits for example as well.

Inputs with the same name

Let's say you've got a list of possible tags and a user can tick the ones that apply, perhaps which programming languagues they know, like so:

<input name="tags" type="checkbox" value="javascript" />
<input name="tags" type="checkbox" value="python" />

With the current solution, you would only get the last selected checkbox as the keys would be overridden in the reduce. However there is a simple fix for this as well.

We check to see if the key (the name attribute on the input) already exists in the result, if it does then use a getAll method which will get a list of results.

/**
 * Creates a json object including fields in the form
 *
 * @param {HTMLElement} form The form element to convert
 * @return {Object} The form data
 */
const getFormJSON = (form) => {
  const data = new FormData(form);
  return Array.from(data.keys()).reduce((result, key) => {
    if (result[key]) {
      result[key] = data.getAll(key)
      return result
    }
    result[key] = data.get(key);
    return result;
  }, {});
};

Similarly, to the file upload before, you'll want to handle only one being ticked, no being ticked or multiple, with something like this.

[result.tags || []].flat();

Examples

Shut up and show me the code.

Interactive
Simple
Full
Bonus: Example Test

Simple Example

<!doctype html>
<html lang="en">

<head>
  <meta charset="utf-8">
</head>

<body>
  <form name="forms" id="forms">
    <label>Whats your username?
      <input name="username" type="text" />
    </label>
    <label>How many years have you been a developer?
      <input name="age" type="number" />
    </label>
    <button type="submit">Submit</button>
  </form>

  <script>
    // get the form element from dom
    const formElement = document.querySelector('form#forms')

    // convert the form to JSON
    const getFormJSON = (form) => {
      const data = new FormData(form);
      return Array.from(data.keys()).reduce((result, key) => {
        result[key] = data.get(key);
        return result;
      }, {});
    };

    // handle the form submission event, prevent default form behaviour, check validity, convert form to JSON
    const handler = (event) => {
      event.preventDefault();
      const valid = formElement.reportValidity();
      if (valid) {
        const result = getFormJSON(formElement);
        console.log(result)
      }
    }

    formElement.addEventListener("submit", handler)
  </script>
</body>

Full Example

<!doctype html>
<html lang="en">

<head>
  <meta charset="utf-8">
</head>

<body>
  <form name="forms" id="forms">
    <label>Whats your username?
      <input name="username" type="text" />
    </label>
    <label>How many years have you been a developer?
      <input name="age" type="number" />
    </label>

    <label>Upload images
      <input name="images" type="file" accept="image/png, image/jpeg" multiple />
    </label>

    <label>Do you know javascript?
      <input name="languages" type="checkbox" value="javascript" />
    </label>
    <label>Do you know python?
      <input name="languages" type="checkbox" value="python" />
    </label>

    <label>Enjoyed this blog?
      <input name="isHappyReader" type="checkbox" />
    </label>

    <button type="submit">Submit</button>
  </form>

  <script>
    // get the form element from dom
    const formElement = document.querySelector('form#forms')

    // convert the form to JSON
    const getFormJSON = (form) => {
      const data = new FormData(form);
      return Array.from(data.keys()).reduce((result, key) => {
        if (result[key]) {
          result[key] = data.getAll(key)
          return result
        }
        result[key] = data.get(key);
        return result;
      }, {});
    };

    // handle the form submission event, prevent default form behaviour, check validity, convert form to JSON
    const handler = (event) => {
      event.preventDefault();
      const valid = formElement.reportValidity();
      if (valid) {
        const result = getFormJSON(formElement);
        // handle one, multiple or no files uploaded
        const images = [result.images].flat().filter((file) => !!file.name)
        // handle one, multiple or no languages selected
        const languages = [result.languages || []].flat();
        // convert the checkbox to a boolean
        const isHappyReader = !!(result.isHappyReader && result.isHappyReader === 'on')

        // use spread function, but override the keys we've made changes to
        const output = {
          ...result,
          images,
          languages,
          isHappyReader
        }
        console.log(output)
      }
    }

    formElement.addEventListener("submit", handler)
  </script>
</body>

Bonus: Example Test

it('should return a JSON representation of a form', () => {
  const form = document.createElement('form');

  const input = document.createElement('input');
  input.name = 'test';
  input.value = 'value';
  form.appendChild(input);

  const number = document.createElement('input');
  number.type = 'number';
  number.name = 'int';
  number.value = '10';
  form.appendChild(number);

  const result = getFormJSON(form);
  expect(result).to.deep.equal({
    test: 'value',
    int: '10',
  });
});

Summary

In summary, you can use what browsers give you to get all the great benefits of HTML Forms, and then convert it to JSON so it's easier to work with the data! I hope this has been useful.

Happy Form building!

Would you be interested in a series of what you can achieve with HTML and Web APIs?
What are you favourite web tips and tricks?

Top comments (13)

NDREAN • Dec 21 '20

you can also try Object.fromEntries([...formData])

Austin Gil • Dec 22 '20

In fact, you don't event need the [...]. You can just do Object.fromEntries(formData) :)

Jordan Finneran • Dec 22 '20

Oh, nice that's even simpler! If you don't have the 'inputs with same name' gotcha.

Austin Gil • Dec 22 '20

Right. Do you have examples of inputs with the same name? I can only think of radio inputs, but in that case, it will only take the one that was selected. You probably know some other use-cases though.

Jordan Finneran • Dec 23 '20

It probably not that common but if you want a list of things a user could check.

For example, if there was a list of languages as checkbox inputs, they could check each one they knew and the output of the Full Example would give you an Array of the languages the user knows.
If you open the Interactive Example in JSFiddle you can see the console output for this :)

NDREAN • Dec 23 '20

I have a question; does this make sense if you use name="lang[js]" and name="lang[py]" ?

Jordan Finneran • Dec 23 '20

You could also do this. It depends what processing you want to do afterwards. :)
I've included both in here jsfiddle.net/8fL3vra1/
Where you can run it, what you get is:

{
  lang[js]: "on",
  lang[py]: "on",
  languages: ["javascript", "python"]
}

My personal preference is to get the list of items but it's completely up to you how you want to achieve it :D

Austin Gil • Dec 23 '20

Oh yeah, that would make sense. I usually give each checkbox its own name, and if they are related, I put them inside a <fieldset>. I'll need to remember this if I'm working with objects and want a list.

Usually I just wrap the data in a URLSearchParams and send it to the backend like that. URLSearchParams will handle the list if there are multiple entries with the same name.

document.querySelector('form').addEventListener('submit', (e) => {
  const formData = new FormData(e.target)
  const params = new URLSearchParams(formData)

  fetch(`https://example.com?${params.toString()}`)

  fetch(`https://example.com`, {
    method: 'POST',
    body: params
  })

  e.preventDefault()
})

NDREAN • Dec 23 '20 • Edited

If you POST, I believe you can do directly body: new FormData(e.target)
if you GET and adjoin a query string, you may indeed need URLSearchParams

Austin Gil • Dec 23 '20

Ah yes. This is true. My only issue with this is using FormData in the request body changes the request headers from the default application/x-www-form-urlencoded to multipart/form-data. This has caused me issues before, so I try to avoid it.

Antonio Pangallo • Dec 22 '20

Hi Jordan, nice post. If you use Reactjs you may be also interested on iusehooks.github.io/usetheform/ . It handles forms state in react and much more.

Jordan Finneran • Dec 23 '20 • Edited

Thank you for the comment. :)
Is this a library you've written?

Antonio Pangallo • Dec 23 '20

Hi, yes I did write it :)

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How to easily convert HTML Form to JSON

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