MobX Easy Form

Simple and performant form library built with MobX.

Works with React, React Native and any other framework that supports MobX.

Motivation

This library is heavily inspired by Formik. I like Formik, but there are some issues with Formik that MobX Easy Form solves.

1. Performace

Forms can get very complicated. Formik can get slow as the form grows as it re-renders the whole form on every keystroke. There are available optimizations but it's complex to setup and can introduce bugs like a field not rendering when it should.

MobX Easy Form solves this issue with MobX. It makes sure to re-render only the components that need to re-render.

You can see the difference in the following GIF. The number of times each component renders is shown below.

2. Validation limitations

Formik forces you to define the validationScheme and initialValues for all your fields at the same time, which means you can't change the validation logic based on the value of some field.

With MobX Easy Form you define each field separately so you can use one field value to initialize another.

Installation

Installing the dependencies

MobX Easy Form depends on mobx, and if you're using React, you'll also need mobx-react-lite (or mobx-react if you support class components). To install these, run

npm install mobx mobx-react-lite

# if you use yarn
yarn add mobx mobx-react-lite

Installing MobX Easy Form

npm install mobx-easy-form

# if you use yarn
yarn add mobx-easy-form

Quick start with React

import { useForm, useField } from "mobx-easy-form";
import { Observer, observer } from "mobx-react";
import { z } from "zod";

export default observer(function Form() {
  // 1. Define each field on its own.
  const firstName = useField({
    initialValue: "",
  });

  const lastName = useField({
    initialValue: "",
  });

  const initials = useField({
    initialValue: "",
    validate(initials) {
      if (
        initials.length === 2 &&
        initials[0] === firstName.state.value?.[0] &&
        initials[1] === lastName.state.value?.[0]
      ) {
        return { error: undefined, parsed: initials };
      }

      return { error: "Wrong initials", parsed: undefined };
    },
  });

  // Parsed type (number) is inferred from the schema. No generics needed.
  const age = useField({
    initialValue: "",
    validationSchema: z.coerce.number(),
  });

  // 2. Collect the fields into a form. The keys you choose here become the
  //    keys of `values`, fully typed: values.age is `number`, the rest `string`.
  const form = useForm({
    fields: { firstName, lastName, initials, age },
    onSubmit({ values }) {
      console.log("Values:", values);
    },
  });

  return (
    <div>
      <h1>User info</h1>

      <Observer>
        {() => {
          return (
            <div>
              <div>First name</div>
              <input
                value={firstName.state.value}
                onChange={(e) => firstName.actions.onChange(e.target.value)}
                onFocus={() => firstName.actions.onFocus()}
                onBlur={() => firstName.actions.onBlur()}
              ></input>
              <div>{firstName.computed.ifWasEverBlurredThenError}</div>
            </div>
          );
        }}
      </Observer>

      <Observer>
        {() => {
          return (
            <div>
              <div>Last name</div>
              <input
                value={lastName.state.value}
                onChange={(e) => lastName.actions.onChange(e.target.value)}
                onFocus={() => lastName.actions.onFocus()}
                onBlur={() => lastName.actions.onBlur()}
              ></input>
              <div>{lastName.computed.ifWasEverBlurredThenError}</div>
            </div>
          );
        }}
      </Observer>

      <Observer>
        {() => {
          return (
            <div>
              <div>Initials</div>
              <input
                value={initials.state.value}
                onChange={(e) => initials.actions.onChange(e.target.value)}
                onFocus={() => initials.actions.onFocus()}
                onBlur={() => initials.actions.onBlur()}
              ></input>
              <div>{initials.computed.ifWasEverBlurredThenError}</div>
            </div>
          );
        }}
      </Observer>

      <Observer>
        {() => {
          return (
            <div>
              <div>Age</div>
              <input
                value={age.state.value}
                onChange={(e) => age.actions.onChange(e.target.value)}
                onFocus={() => age.actions.onFocus()}
                onBlur={() => age.actions.onBlur()}
              ></input>
              <div>{age.computed.ifWasEverBlurredThenError}</div>
            </div>
          );
        }}
      </Observer>

      <Observer>
        {() => {
          return (
            <button
              onClick={form.actions.submit}
              disabled={form.computed.isError && form.state.submitCount > 0}
            >
              SUBMIT ({form.computed.isError ? "invalid" : "valid"})
            </button>
          );
        }}
      </Observer>
    </div>
  );
});

Migrating to V4

V4 changes how fields and forms are connected so that onSubmit's values (and rawValues, and fields) are fully typed automatically — no generics, no manual type declarations.

What changed

In V3 you created the form first, then each field registered itself into the form by receiving form and an id. The form had no idea what fields it would end up with, so values was Record<string, unknown>.

In V4 you do the opposite: create the fields first, then pass them to the form as a fields record. The keys of that record become the keys of values, and their types are inferred from each field.

| V3 | V4 | | --- | --- | | createForm({ onSubmit }) first, fields self-register | Fields first, then createForm({ fields, onSubmit }) | | createField({ id, form, initialValue }) | createField({ initialValue }) — no id, no form | | Field key comes from id | Field key comes from the fields record key | | form.actions.add(field) | removed — pass fields to createForm instead | | field.state.id | removed | | values is Record<string, unknown> | values is fully typed from the field record |

Before (V3)

const form = useForm({
  onSubmit({ values }) {
    // values.age is `unknown`
    console.log(values);
  },
});

const name = useField({ id: "name", form, initialValue: "" });
const age = useField({
  id: "age",
  form,
  initialValue: "",
  validationSchema: z.coerce.number(),
});

After (V4)

const name = useField({ initialValue: "" });
const age = useField({
  initialValue: "",
  validationSchema: z.coerce.number(),
});

const form = useForm({
  fields: { name, age },
  onSubmit({ values }) {
    values.name; // string
    values.age; // number
  },
});

Step-by-step

1. Move field creation above the form. Fields no longer need the form, so create them first.
2. Remove form and id from every createField / useField call. The record key replaces id.
3. Add a fields record to createForm / useForm. Use the names you previously passed as id as the record keys: fields: { name, age }.
4. Replace form.actions.add(field) calls — there is no add anymore; everything is passed up front via fields.
5. Replace any field.state.id usage. The id no longer exists on the field; use the key under which you stored it in the form.

Note: the field set is now closed — it's whatever you pass to createForm. There is no runtime add/remove. This is what makes values fully typed. Dynamic forms (e.g. a variable number of rows) are intended to be served by a future createFieldArray primitive that nests as a single entry in the record.

Typed values

values.<key> is typed as the field's parsed/output type, and rawValues.<key> as its input type:

const age = useField({ initialValue: "", validationSchema: z.coerce.number() });

const form = useForm({
  fields: { age },
  onSubmit({ values, rawValues }) {
    values.age; // number  (parsed output)
    rawValues.age; // string  (raw input held in state.value)
  },
});

onSubmit only runs when the form is valid, so values.<key> drops the "failed validation" undefined and gives you the clean parsed type. If a field's output type is genuinely optional (e.g. its schema/validate can return undefined as a valid result), that undefined is preserved.

React Example on CodeSandbox

https://codesandbox.io/s/mobx-easy-form-example-xn3ss

React Native Example on Expo Snack

https://snack.expo.dev/@hrastnik/react-native---mobx-easy-form

Getting started

First create your fields. Each field is a standalone object — you only need an initialValue.

const firstName = useField({
  initialValue: "",
});

Then create the form, passing all your fields as a record. The keys you pick here are how you'll access the fields later, and they're also what drives the type inference for onSubmit.

const form = useForm({
  fields: { firstName },
  async onSubmit({ values }) {
    // `values` is fully typed from the field record.
    console.log(values.firstName); // string
  },
});

The key is also how you access the field through the form object — it's the exact same instance you passed in.

form.fields.firstName === firstName; // true

Because each field is created on its own before the form, you can use one field's value inside another field's validation (e.g. a "confirm password" field).

const password = useField({ initialValue: "" });
const confirmPassword = useField({
  initialValue: "",
  validate: (value) =>
    value === password.state.value
      ? { error: undefined, parsed: value }
      : { error: "Passwords must match", parsed: undefined },
});

const form = useForm({ fields: { password, confirmPassword }, onSubmit });

You can pass a validation function to useField, or use any validator that implements the Standard Schema spec — including Zod, Valibot, and ArkType. Yup is also supported via its validateSync interface.

Using Zod (or any Standard Schema validator)

import { z } from "zod";

const age = useField({
  initialValue: "",
  validationSchema: z.coerce.number().min(0, "Age must be positive"),
});

The library infers the input type from initialValue and the parsed/output type from validationSchema, so you usually don't need explicit generics. In this example age.state.value is string and age.computed.parsed is number.

Using Yup

import { number } from "yup";

const age = useField({
  initialValue: "",
  validationSchema: number()
    .typeError("Age should be a number")
    .required("Age is required"),
});

MobX Easy Form uses the validation function for two things:

1. To check if the value is in the right format.
2. To parse the value and convert it to something usable.

In the example above, the field state will hold the string value of a number - e.g. "42", but the schema will convert it to a number 42 that we can access through age.computed.parsed and is also the value we get in onSubmit({ values }).

Synchronous validation only

Validation runs inside MobX computed values, so it must be synchronous. If a Standard Schema validator returns a Promise (for example, a Zod schema with .refine(async ...)), accessing field.computed.parsed or field.computed.error will throw. For async checks (e.g. server-side uniqueness), run them at the application level and call field.actions.setError(...) with the result.

IMPORTANT!

The useForm and useField are React hooks that use createForm and createField and cache the result so that React doesn't re-create the form on each render. In other words useForm and useField make sure the form and fields only get created once. If you want to re-create the fields you can pass a dependency array as the second parameter to useForm and useFields.

Render

We can now render the inputs. A great performance optimization is to use the <Observer> component from mobx-react to limit the renders to only the input that needs to change. Alternatively you can create your own input component with the observer HOC.

Each field has the state,computed and actions props.

<Observer>
  {() => {
    return (
      <div>
        <div>First name</div>
        <input
          value={firstName.state.value}
          onChange={(e) => firstName.actions.onChange(e.target.value)}
          onFocus={() => firstName.actions.onFocus()}
          onBlur={() => firstName.actions.onBlur()}
        ></input>
        <div>{firstName.computed.ifWasEverBlurredThenError}</div>
      </div>
    );
  }}
</Observer>

API

useForm / createForm

| parameter | type | required | description | | ---------- | ---------------------------- | -------- | ---------------------------------------------------------------------------------------------------------------------------------------- | | fields | Record<string, Field>; | true | The complete record of fields for this form. The keys become the keys of values/rawValues/fields, and drive the type inference. | | onSubmit | (props: OnSubmitArg) => any; | true | Function that will be ran when form.actions.submit is called if the form is valid. |

Returns a Form instance.

OnSubmitArg

| parameter | type | description | | ----------- | --------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | fields | typeof fields | The exact field record passed to createForm/useForm — same instances, keyed by the same keys. | | rawValues | typed by field input | The raw input value (field.state.value) of every field, keyed by field key. Typed automatically from each field's input type. | | values | typed by field output | The parsed/output value of every field, keyed by field key. Typed automatically from each field's parsed type (the value returned by validate / the validation schema). |

useField / createField

| parameter | type | required | description | | ------------------ | ------------------------------------------------------------------------------------------- | -------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | initialValue | any | required | The initial value of the field. If you're using TypeScript, note that this field will be used to type the values you can set through onChange, so for optional fields consider using a cast to add the undefined type. (initalValue: "" as "" \| undefined) | | validationSchema | StandardSchemaV1 | Yup.ValidationSchema | optional | A schema used to validate and parse the field value. Accepts any Standard Schema validator (Zod, Valibot, ArkType, etc.) or a Yup schema. You can only set one of validationSchema and validate. | | validate | (value: any) => { error?: undefined, parsed: any } | { error: string, parsed?: undefined } | optional | Validation function used to validate and parse the value. It receives the value and must return and object with either error or parsed set | | initialError | string | optional | Initial error for the field |

Returns a Field instance

Field

| parameter | type | description | | ------------------------------------ | ----------------------------- | ------------------------------------------------------------------------------------------------------ | | state.errorOverride | string | undefined | Custom error message set on the field | | state.isFocused | boolean | true if the field is focused, false otherwise | | state.value | any | Current value of the field | | state.wasEverBlurred | string | true if the field was ever blurred, false otherwise | | state.wasEverFocused | string | true if the field was ever focused, false otherwise | | computed.error | string | undefined | Custom error message set on the field | | computed.ifWasEverBlurredThenError | string | undefined | if the field was ever blurred returns the value of computed.error, otherwise returns undefined | | computed.ifWasEverFocusedThenError | string | undefined | if the field was ever focused returns the value of computed.error, otherwise returns undefined | | computed.isDirty | boolean | true if value is different from initialValue, false otherwise | | computed.parsed | any | Parsed value of the field. The parsed value returned by validate or cast using the validation schema | | actions.onBlur | () => void | Sets state.isFocused to false. Should be passed to text inputs onBlur | | actions.onChange | (value: any) => void | Sets state.value to the provided value. Additionally, it clears the errorOverride if it was set | | actions.onFocus | () => void | Sets state.isFocused to true. Should be passed to text inputs onFocus | | actions.setError | (string | undefined) => void | Sets state.errorOverride to the provided value |

Form

| parameter | type | description | | ----------------------------------- | ----------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | state.isSubmitting | boolean | true if the form is submitting, when the onSubmit function is async | | state.submitCount | number | Number of times the form was submitted | | state.valuesAtLastSubmit | string | undefined | Used internally. You shouldn't use this | | computed.errorList | [string | undefined] | An array of errors for all the fields | | computed.isChangedSinceLastSubmit | boolean | true if any field has changed since last submit, false otherwise | | computed.isDirty | boolean | true if any field is dirty (value different from initialValue) | | computed.isError | boolean | true if any field has error different from undefined | | computed.isValid | boolean | true if isError is false, false otherwise | | computed.valueList | any[] | An array of all the values. Used internally, you shouldn't use this. | | actions.submit | (value: any) => Promise | Sets isSubmitting to true, increases submitCount, for each field, sets the wasEverFocused and wasEverBlurred to true. Then, if isError is true does nothing, otherwise runs the onSubmit function |