hunk.code_window: #### `resetForm`
   Because we introduced the new `initialErrors`, `initialTouched`, `initialStatus` props, `resetForm`'s signature has changed since there is more stuff to _reset_. It now accepts the next initial state of Formik (instead of just the next initial values).
   ### `validate`
   As you may know, you can return a Promise of a validation error from `validate`. In 1.x, it didn't matter if this promise is resolved or rejected as in both cases the payload of the promise was interpreted as the validation error. In 2.x, rejection will be interpreted as an actual exception and it won't update the form error state. Any validation function that returns a rejected promise of errors needs to be adjusted to return a resolved promise of errors instead.
  
hunk.labels: keep keep keep keep add keep keep keep
hunk.after_edit: ### `setError`
   Please use Formik's `setStatus(status)` instead. It works identically.
  
hunk.file_path: packages/formik/MIGRATING-v2.md
hunk.type: add
file: ---
  id: fastfield
  title: <FastField />
  custom_edit_url: https://github.com/jaredpalmer/formik/edit/master/docs/api/fastfield.md
  ---
  
  ## Before we start
  
  `<FastField />` is meant for performance _optimization_. However, you really do not need to use it until you do. Only proceed if you are familiar with how React's [`shouldComponentUpdate()`](https://reactjs.org/docs/react-component.html#shouldcomponentupdate) works. You have been warned.
  
  **No. Seriously. Please review the following parts of the official React documentation before continuing**
  
  - [React `shouldComponentUpdate()` Reference](https://reactjs.org/docs/react-component.html#shouldcomponentupdate)
  - [`shouldComponentUpdate` in Action](https://reactjs.org/docs/optimizing-performance.html#shouldcomponentupdate-in-action)
  
  ## Overview
  
  `<FastField />` is an optimized version of `<Field />` meant to be used on large forms (~30+ fields) or when a field has very expensive validation requirements. `<FastField />` has the same exact API as `<Field>`, but implements `shouldComponentUpdate()` internally to block all additional re-renders unless there are direct updates to the `<FastField />`'s relevant parts/slice of Formik state.
  
  For example, `<FastField name="firstName" />` will only re-render when there are:
  
  - Changes to `values.firstName`, `errors.firstName`, `touched.firstName`, or `isSubmitting`. This is determined by shallow comparison. Note: dotpaths are supported.
  - A prop is added/removed to the `<FastField name="firstName" />`
  - The `name` prop changes
  
  Other than for these aforementioned situations, `<FastField />` will not re-render when other parts of of Formik state change. However, all updates triggered by a `<FastField />` will trigger re-renders to other "vanilla" `<Field />` components.
  
  ## When to use `<FastField />`
  
  **If a `<Field />` is "independent" of all other `<Field />`'s in your form, then you can use `<FastField />`**.
  
  More specifically, if the `<Field />` does not change behavior or render anything that is based on updates to another `<Field />` or `<FastField />`'s slice of Formik state AND it does not rely on other parts of top-level `<Formik />` state (e.g. `isValidating`, `submitCount`), then you can use `<FastField />` as a drop-in replacement to `<Field />`.
  
  ## Example
  
  ```jsx
  import React from 'react';
  import { Formik, Field, FastField, Form } from 'formik';
  import * as Yup from 'yup';
  
  const Basic = () => (
    <div>
      <h1>Sign Up</h1>
      <Formik
        initialValues={{
          firstName: '',
          lastName: '',
          email: '',
        }}
        validationSchema={Yup.object().shape({
          firstName: Yup.string().required(),
          middleInitial: Yup.string(),
          lastName: Yup.string().required(),
          email: Yup.string()
            .email()
            .required(),
        })}
        onSubmit={values => {
          setTimeout(() => {
            alert(JSON.stringify(values, null, 2));
          }, 500);
        }}
        render={formikProps => (
          <Form>
            {/** This <FastField> only updates for changes made to
             values.firstName, touched.firstName, errors.firstName */}
            <label htmlFor="firstName">First Name</label>
            <FastField name="firstName" placeholder="Weezy" />
  
            {/** Updates for all changes because it's from the
             top-level formikProps which get all updates */}
            {form.touched.firstName &&
              form.errors.firstName && <div>{form.errors.firstName}</div>}
  
            <label htmlFor="middleInitial">Middle Initial</label>
            <FastField name="middleInitial" placeholder="F">
              {({ field, form, meta }) => (
                <div>
                  <input {...field} />
                  {/**
                   * This updates normally because it's from the same slice of Formik state,
                   * i.e. path to the object matches the name of this <FastField />
                   */}
                  {meta.touched ? meta.error : null}
  
                  {/** This won't ever update since it's coming from
                   from another <Field>/<FastField>'s (i.e. firstName's) slice   */}
                  {form.touched.firstName && form.errors.firstName
                    ? form.errors.firstName
                    : null}
  
                  {/* This doesn't update either */}
                  {form.submitCount}
  
                  {/* Imperative methods still work as expected */}
                  <button
                    type="button"
                    onClick={form.setFieldValue('middleInitial', 'J')}
                  >
                    J
                  </button>
                </div>
              )}
            </FastField>
  
            {/** Updates for all changes to Formik state
             and all changes by all <Field>s and <FastField>s */}
            <label htmlFor="lastName">LastName</label>
            <Field name="lastName" placeholder="Baby">
              {( }) => (
                <div>
                  <input {...field} />
                  {/**  Works because this is inside
                   of a <Field/>, which gets all updates */}
                  {form.touched.firstName && form.errors.firstName
                    ? form.errors.firstName
                    : null}
                </div>
              )}
            </Field>
  
            {/** Updates for all changes to Formik state and
             all changes by all <Field>s and <FastField>s */}
            <label htmlFor="email">Email</label>
            <Field name="email" placeholder="jane@acme.com" type="email" />
  
            <button type="submit">Submit</button>
          </Form>
        )}
      />
    </div>
  );
  ```
  