Add Formik's Validation Schema As A Function as a react til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
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 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_715 TILs and counting..._
+_716 TILs and counting..._
 
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@@ -486,6 +486,7 @@ _715 TILs and counting..._
 - [Enforce Specific Values With PropTypes](react/enforce-specific-values-with-proptypes.md)
 - [Force A Component To Only Have One Child](react/force-a-component-to-only-have-one-child.md)
 - [Forcing A Child Remount With The Key Prop](react/forcing-a-child-remount-with-the-key-prop.md)
+- [Formik's Validation Schema As A Function](react/formiks-validation-schema-as-a-function.md)
 - [Inactive And Active Component Styles With Radium](react/inactive-and-active-component-styles-with-radium.md)
 - [Inline Style Attributes Should Be Camel Cased](react/inline-style-attributes-should-be-camel-cased.md)
 - [Mapping Over One Or Many Children](react/mapping-over-one-or-many-children.md)

react/formiks-validation-schema-as-a-function.md

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+# Formik's Validation Schema As A Function
+
+The most straightforward way to use
+[Formik](https://jaredpalmer.com/formik)'s `validationSchema` is to provide
+it with a [Yup](https://github.com/jquense/yup) object defining your form's
+validations.
+
+```javascript
+const MyComponent = withFormik({
+  // ...
+
+  validationSchema: yup.object().shape({
+    email: yup.string().required(),
+    feedback: yup.string().required(),
+  }),
+  
+  // ...
+})(MyForm);
+```
+
+There may be a point at which you need access to the `props` being passed
+to `MyComponent` in order to construct the proper set of validations.
+Formik supports this by allowing `validationSchema` to be a function.
+
+```javascript
+const MyComponent = withFormik({
+  // ...
+
+  validationSchema: (props) => {
+    let emailSchema;
+    if(props.allowAnonymous) {
+      emailSchema = yup.string();
+    } else {
+      emailSchema = yup.string().required();
+    }
+
+    return yup.object().shape({
+      email: emailSchema,
+      feedback: yup.string().required(),
+    });
+  },
+  
+  // ...
+})(MyForm);
+```
+
+When `validationSchema` is a function, its first argument is the set of
+props passed to that component.