FormikFields

An extension to the great formik-library.

Motivation

The main goal is to keep the simplicity of formik's <Field>-component and to remove the magic introduced by the implicit context and the name-string. In addition, when updating one field, you do not want to have to render all the others again.

Installation

FormikFields is available as an npm-package and has formik as peer-dependency:

yarn add formik formik-fields
# or npm
npm install formik formik-fields

Example

import * as React from 'react';
import { FormikFields } from 'formik-fields';
import { FormikFieldInput } from './your-custom-component';

interface MyFormValues {
  email: string;
  name: string;
}

export const MyForm = () => (
  <FormikFields<MyFormValues>
    fields={{
      name: {
        initialValue: '',
        validate: val => !val && 'Name should not be empty'
      },
      email: { initialValue: '' }
    }}
    onSubmit={(values: MyFormValues) => console.log(values)}
    render={(fields, formikBag) => (
      <form onSubmit={formikBag.handleSubmit}>
        <FormikFieldInput field={fields.email} />
        <FormikFieldInput field={fields.name} />
        <button type="submit">submit</button>
      </form>
    )}
  />
);

• TypeScript-example on CodeSandbox
• React-JSX example on CodeSandbox

Table of Contents

• <FormikFields>-API
  • fields
  • FieldDefinition
  • render-prop
    • fieldsState
    • formikProps
• TypeScript
  • usage without TypeScript
• advanced
  • custom input example
  • formik under the hood
  • Field-Validators and form-submission
  • Validation and performance

<FormikFields>-API

The <FormikFields>-Component accepts following props:

fields

Type: { [fieldName]: FieldDefinition }

const formikFieldDefinition = {
  name: {
    initialValue: '',
    validate: val => !val && 'Name should not be empty'
  },
  email: { initialValue: '' }
};

FieldDefinition

Name	Type	Description
initialValue (required)	Value	Initial value when the form is first mounted.
validate	(val: Value) => any	If not falsy, the fields error-prop is set with the computed value from this function. See formik's How Form Submission Works to understand how validation works.

render-prop

type: (fieldsState, formikProps) => ReactNode

You can also use the children-prop, for more usage about the render-prop-pattern head over the the react-documentation.

fieldsState

type : { [fieldName]: FieldState }

FieldState is an Object with following members:

Name	Type	Description
name	string	equal to the corresponding key in the fieldsState-Object
value	FormValue[name]	current value, on mount equal to initialValue in the FieldDefinition
isTouched	boolean (default: false)	result from setIsTouched or handleBlur
error	any	result from the validate-function in the FieldDefinition
setValue	(val: FormValue[name], shouldValidate = true) => void	set value and re-evaluate your validators, calls internally formik's setFieldValue
setIsTouched	(isTouched: boolean, shouldValidate = true) => void	set isTouched and re-evaluate your validators, calls internally formik's setFieldTouched
handleChange	(e: React.ChangeEvent) => void	calls setValue with the extracted value from an input-onChange-callback
handleBlur	() => void	calls setIsTouched(true)

formikProps

The second callback-parameter contains all props from Formik's render-prop-callback, visit their documentation for more information.

TypeScript

This project is written in TypeScript and so typings should always be up-to-date. You should always enhance the <FormikFields>-Component with your form-values-Interface (since TypeScript 2.9 you can do this inline):

interface MyFormValues {
  email: string;
  name: string;
}

export const MyForm = () => <FormikFields<MyFormValues> {/*...*/} />

Now your fields and render-prop-parameters are enhanced with your specific field-names and -types.

usage without TypeScript

You can use FormikFields as well as formik without TypeScript (via ES6-import or require). However, you lose the advantages of the types in your forms (with TS the fields are checked against the initial definitions of the form-values-structure and so you can only access defined fields).

advanced

custom input example

A minimum implementation of a FormikFieldInput in TypeScript could look like this:

import { PureComponent } from 'react';
import { FormikFieldState } from 'formik-fields';

interface FormikFieldInputProps {
  field: FormikFieldState<string>;
}

// Use React.PureComponent component to take advantage of the optimized fields
export class FormikFieldInput extends PureComponent<FormikFieldInputProps> {
  render() {
    const { field, ...props } = this.props;

    return (
      <div>
        <input
          onChange={field.handleChange}
          onBlur={field.handleBlur}
          value={field.value}
          {...props}
        />
        {field.isTouched && field.error && <div>({field.error})</div>}
      </div>
    );
  }
}

formik under the hood

Since this is only an extension, <FormikFields> accepts all props of the actual <Formik> component (except Component and initialValues, which is overwritten by your field-definitions).

When you define Formik's validate-prop, the results of the specifically defined validate function are merged with the results of the field-validations (where the specific validate function overwrites the values of the field-validations if they have the same keys).

Field-Validators and form-submission

If you define a field-level-validate-function via the FieldDefinition, the result of your function can prevent your form to submit:

If this function returns a falsy value, this field will not prevent your form to submit. Otherwise, the form validation corresponds to that of Formik, see How Form Submission Works in their documentation.

for advanced validations you can use formik's validate-prop

Validation and performance

your field-validator-function HAVE TO be always a pure function with only one parameter (the field-value itself). Under this assumption, the results of the validations are memoized. The fields are only renewed if there are actual changes. So you can use Reacts PureComponent to improve the performance of your forms.