Add Update Formik Initial Values When Props Change as a react til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_828 TILs and counting..._
+_829 TILs and counting..._
 
 ---
 
@@ -567,6 +567,7 @@ _828 TILs and counting..._
 - [Spelunking Through Components With Enzyme's Dive](react/spelunking-through-components-with-enzymes-dive.md)
 - [Sync Your react-router State With Redux](react/sync-your-react-router-state-with-redux.md)
 - [Test Files In create-react-app](react/test-files-in-create-react-app.md)
+- [Update Formik Initial Values When Props Change](react/update-formik-initial-values-when-props-change.md)
 - [Upgrading To The Latest React In CodeSandbox](react/upgrading-to-the-latest-react-in-codesandbox.md)
 - [Use A Ref To Autofocus An Input](react/use-a-ref-to-autofocus-an-input.md)
 - [Use React 16 With Gatsby](react/use-react-16-with-gatsby.md)

react/update-formik-initial-values-when-props-change.md

@@ -0,0 +1,44 @@
+# Update Formik Initial Values When Props Change
+
+When a [Formik](https://jaredpalmer.com/formik/) form mounts, whatever the
+initial values are set to is what they will be. Even if the initial values are
+computed from props, those props changing will not affect `initialValues` after
+mount.
+
+```javascript
+const ZipForm = ({ currentZip }) => {
+  return (
+    <Formik
+      initialValues={{ zip: currentZip }}
+      onSubmit={(values, actions) => {
+        // do stuff
+      }}
+      ...
+```
+
+If we are fetching the user's saved zip code asynchronously from a server while
+the form is first being rendered, then `currentZip` will start as an empty
+value. Once the async request comes back and `currentZip` is set, we won't see
+the form update the `zip` field.
+
+There was a time when you would have to jump through some hoops to make sure
+the freshest prop value made it into the form. Now, Formik provides a handier
+mechanism -- the `enableReinitialize` prop.
+
+```javascript
+const ZipForm = ({ currentZip }) => {
+  return (
+    <Formik
+      initialValues={{ zip: currentZip }
+      enableReinitialize
+      onSubmit={(values, actions) => {
+        // do stuff
+      }}
+      ...
+```
+
+By setting `enableReinitialize` to true, we are telling Formik that any prop
+changes that flow into the `initialValues` object should cause those values to
+be _reinitialized_.
+
+See a [live example](https://codesandbox.io/s/sad-mendeleev-4dbbp).