Add Check The Password Confirmation With Yup as a javascript til

javascript/check-the-password-confirmation-with-yup.md

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+# Check The Password Confirmation With Yup
+
+The [Yup](https://github.com/jquense/yup) library makes it easy to validate
+individual values in a JavaScript object. A common situation when
+implementing a Sign Up form is asking the user to input their password twice
+and then the app can make sure they match.
+
+To do this, we need the validation of our `passwordConfirmation` value to
+reach outside of itself to make a comparison with the `password` value. This
+can be done with Yup's `ref` function.
+
+```javascript
+import * as Yup from 'yup';
+
+validationSchema: Yup.object({
+  password: Yup.string().required('Password is required'),
+  passwordConfirmation: Yup.string()
+     .oneOf([Yup.ref('password'), null], 'Passwords must match')
+});
+```
+
+We are able to reference the value of `password` with `ref`. We use the
+`oneOf` function to ensure that `passwordConfirmation` either matches
+`password` or if it is left blank and matches `null` then it passes the
+validation for the time being. The second argument to `oneOf` is a custom
+validation message when this validation fails.
+
+[source](https://github.com/jaredpalmer/formik/issues/90)