Add Custom Type Checking Error Messages With Yup as a javascript til

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/).
 
-_635 TILs and counting..._
+_636 TILs and counting..._
 
 ---
 
@@ -227,6 +227,7 @@ _635 TILs and counting..._
 - [Computed Property Names In ES6](javascript/computed-property-names-in-es6.md)
 - [Create An Array Containing 1 To N](javascript/create-an-array-containing-1-to-n.md)
 - [Create Bootstrapped Apps With Yarn](javascript/create-bootstrapped-apps-with-yarn.md)
+- [Custom Type Checking Error Messages With Yup](javascript/custom-type-checking-error-messages-with-yup.md)
 - [Default And Named Exports From The Same Module](javascript/default-and-named-exports-from-the-same-module.md)
 - [Destructuring The Rest Of An Array](javascript/destructuring-the-rest-of-an-array.md)
 - [Enable ES7 Transforms With react-rails](javascript/enable-es7-transforms-with-react-rails.md)

javascript/custom-type-checking-error-messages-with-yup.md

@@ -0,0 +1,25 @@
+# Custom Type Checking Error Messages With Yup
+
+In [Yup Schemas Are Validated
+Asynchronously](https://github.com/jbranchaud/til/blob/master/javascript/yup-schemas-are-validated-asynchronously.md),
+I showed how to create a simple schema that allows you to enforce that a
+value is a number.
+
+```javascript
+const numSchema = yup.number();
+```
+
+If we use this schema to validate something that isn't a number, Yup will
+provide a lengthy default message. Here is what we get if I validate against
+`'hey'`:
+
+> this must be a `number` type, but the final value was: `NaN` (cast from
+> the value `"hey"`).
+
+This value isn't necessarily suitable for displaying to a user. We can
+customize the type checking error message by redefining our schema with the
+`typeError()` function:
+
+```javascript
+const numSchema = yup.number().typeError("Invalid number");
+```