yup - is there any way to set the default value for a string field to be something without defining it for each one - yup

Question

I want that every time I use yup.string(), it will add a specific default value for it
for example:
const schema = yup.object({
text: yup.string()// I want it to also do .default('some string') in the background,
});
or - another option - is there any way to set the default value after creating the scheme? something like setDefault('text', 'some string')

Answer 1

The closest solution I came across to solve your issue is extending your string with a custom method that implements your needs. To do that you need to use addMethod from yup:
import { addMethod, string } from 'yup';
addMethod(string, 'append', function append(appendStr) {
return this.transform((value) => `${value}${appendStr}`);
});
Now, you can use your custom method (append) and apply it to any string you want:
string().append('~~~~').cast('hi'); // 'hi~~~~'
If you want to add the custom method to all your schema types like date, number, etc..., you need to extend the abstract base class Schema:
import { addMethod, Schema } from 'yup';
addMethod(Schema, 'myCustomMethod', ...)
Extra
For Typescript
In your type definition file, you need to declare module yup with your custom method's arguments and return types:
// globals.d.ts
import { StringSchema } from "yup";
declare module 'yup' {
interface StringSchema<TType, TContext, TDefault, TFlags> {
append(appendStr: string): this;
}
}
Unknow behavior for transform method
While I was trying to extend the functionality of the date schema with a custom method that transform the date that user enters from DD-MM-YYY to YYYY-MM-DD, the custom method broke after I used it with other methods like min, max for example.
// `dayMonthYear` should transform "31-12-2022"
// to "2022-12-31" but for some reason it kept
// ignoring the `cast` date and tried to transform
// `1900` instead!
Yup.date().dayMonthYear().min(1900).max(2100).required().cast("31-12-2022") // error
To work around this issue, I appended my custom method at the end of my schema chain:
Yup.date().min(1900).max(2100).required().cast("31-12-2022").dayMonthYear() // works as expected
This issue is mentioned in this GH ticket which I recommend going through it as it's going more in-depth on how to add custom methods with Typescript.
References
addMethod
Extending built-in schema with new methods
Example of addMethod in Typescript (GH ticket)