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

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

@@ -0,0 +1,48 @@
+# 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.