> For the complete documentation index, see [llms.txt](https://ploegert.gitbook.io/til/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ploegert.gitbook.io/til/programmy/javascript/custom-type-checking-error-messages-with-yup.md).

# 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 we 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");
```