<FastField />

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() works. You have been warned.

No. Seriously. Please review the following parts of the official React documentation before continuing

• React shouldComponentUpdate() Reference
• 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

1 import React from 'react';
2 import { Formik, Field, FastField, Form } from 'formik';
3 import * as Yup from 'yup';
4 
5 const Basic = () => (
6   <div>
7     <h1>Sign Up</h1>
8     <Formik
9       initialValues={{
10         firstName: '',
11         lastName: '',
12         email: '',
13       }}
14       validationSchema={Yup.object().shape({
15         firstName: Yup.string().required(),
16         middleInitial: Yup.string(),
17         lastName: Yup.string().required(),
18         email: Yup.string().email().required(),
19       })}
20       onSubmit={values => {
21         setTimeout(() => {
22           alert(JSON.stringify(values, null, 2));
23         }, 500);
24       }}
25       render={formikProps => (
26         <Form>
27           {/** This <FastField> only updates for changes made to
28            values.firstName, touched.firstName, errors.firstName */}
29           <label htmlFor="firstName">First Name</label>
30           <FastField name="firstName" placeholder="Weezy" />
31 
32           {/** Updates for all changes because it's from the
33            top-level formikProps which get all updates */}
34           {form.touched.firstName && form.errors.firstName && (
35             <div>{form.errors.firstName}</div>
36           )}
37 
38           <label htmlFor="middleInitial">Middle Initial</label>
39           <FastField name="middleInitial" placeholder="F">
40             {({ field, form, meta }) => (
41               <div>
42                 <input {...field} />
43                 {/**
44                  * This updates normally because it's from the same slice of Formik state,
45                  * i.e. path to the object matches the name of this <FastField />
46                  */}
47                 {meta.touched ? meta.error : null}
48 
49                 {/** This won't ever update since it's coming from
50                  from another <Field>/<FastField>'s (i.e. firstName's) slice   */}
51                 {form.touched.firstName && form.errors.firstName
52                   ? form.errors.firstName
53                   : null}
54 
55                 {/* This doesn't update either */}
56                 {form.submitCount}
57 
58                 {/* Imperative methods still work as expected */}
59                 <button
60                   type="button"
61                   onClick={form.setFieldValue('middleInitial', 'J')}
62                 >
63                   J
64                 </button>
65               </div>
66             )}
67           </FastField>
68 
69           {/** Updates for all changes to Formik state
70            and all changes by all <Field>s and <FastField>s */}
71           <label htmlFor="lastName">LastName</label>
72           <Field name="lastName" placeholder="Baby">
73             {() => (
74               <div>
75                 <input {...field} />
76                 {/**  Works because this is inside
77                  of a <Field/>, which gets all updates */}
78                 {form.touched.firstName && form.errors.firstName
79                   ? form.errors.firstName
80                   : null}
81               </div>
82             )}
83           </Field>
84 
85           {/** Updates for all changes to Formik state and
86            all changes by all <Field>s and <FastField>s */}
87           <label htmlFor="email">Email</label>
88           <Field name="email" placeholder="jane@acme.com" type="email" />
89 
90           <button type="submit">Submit</button>
91         </Form>
92       )}
93     />
94   </div>
95 );