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

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

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+# 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");
+```