Declarative Formik component for side-effects. Use with caution. Please. I. beg. you.

npm install formik-effect --save

Note: this has peer dependencies of prop-types, react, and formik (obvs)

Formik is an uncontrolled component. However, there are times when you want to trigger a side effect based on a state change. By design, Formik does not expose a prop for you to do this because I'm terrified as to how it would be abused--it encourages people to attempt to "sync" Formik's state in elsewhere (cough like in a Redux store cough cough). Anyways, please don't do that. You never ever ever ever want to store the same state in 2 places in a React application because it is almost impossible to keep them in sync unless you are a goddamn jedi. You may think it's working, and high five a teammate, but you are a just a lifecycle method away from lots and lots of pain and I can guarantee you are not considering all the edge cases. Sooooo....

SAY IT WITH ME:

"I WILL NOT USE THIS TO STORE FORMIK STATE ELSEWHERE IN MY APP."
"I WILL NOT USE THIS TO STORE FORMIK STATE ELSEWHERE IN MY APP."
"I WILL NOT USE THIS TO STORE FORMIK STATE ELSEWHERE IN MY APP."

Import the <Effect > component and put it inside any Formik form. It renders null! Pass it an onChange() function and it will be called whenever your Formik form's state updates.

import React from 'react'
import { Formik, Field, Form } from 'formik'
import { Effect } from 'formik-effect'

export const Signup = () =>
  <div>
    <h1>My Cool Form with a SideEffect</h1>
    <Formik
      onSubmit={values => console.log(values)}
      initialValues={{ firstName: '', lastName: '', email: '' }}
      render={props =>
        <Form className="whatever">
          <Effect onChange={(currentFormikState, nextFormikState) => {
               // do whatevs
               // FYI if you alter state it will cause another render
            }} 
          />
          <Field name="firstName" placeholder="First Name" />
          <Field name="lastName" placeholder="Last Name" />
          <Field name="email" type="email" placeholder="Email Address" />
          <button type="submit">Submit</button>
         
        </Form>}
    />
  </div>;

Only one!

Put your side effect here.

FormikState includes:

• values
• errors
• touched
• isSubmitting

Under the hood this calls componentWillReceiveProps(). When Formik refactors for React 16.3, it will use componentDidUpdate. Either way, it does shallow comparison on context with triple equals, which may not be what you want. Luckily, this whole component is like 500 bytes so you could just copy pasta it into your app.

When Formik is updated to React 16.3, this library will need to be updated for use without PropTypes.

• Jared Palmer @jaredpalmer