How to Fix undefined Values in Yup's .when Method for String Validation

Learn how to correctly pass string values into Yup's `.when` method to enhance your validation logic in JavaScript and React applications.
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Understanding the undefined Issue in Yup's .when Method

When working with form validation in JavaScript, especially with libraries like Formik and Yup, you might encounter an issue where the value you expect appears as undefined. This can be frustrating, but understanding why this happens will help you fix it swiftly.

The Problem

In your case, you've observed that the iscountryCode value returns as undefined when you pass it into .when(). Despite your console log statement showing that the value is indeed 'BE', the schema setup appears to be incorrect. Let’s unpack your code and the solution step by step.

Example Code

Here is a glimpse of your original code for clarity:

[[See Video to Reveal this Text or Code Snippet]]

This setup leads to your main issue: iscountryCode should not be passed directly. Instead, it should be treated as a string literal.

The Solution

1. Use String Literals

Instead of passing the variable iscountryCode, you need to pass the string 'iscountryCode'. Here’s how you can tweak your code:

[[See Video to Reveal this Text or Code Snippet]]

2. Include Required Fields in Your Schema

For the .when() method to work correctly, you also need to ensure that the field you're referencing, iscountryCode, is defined in your schema. Therefore, your schema should incorporate the iscountryCode field at the sibling level with zip. Here's an updated example:

[[See Video to Reveal this Text or Code Snippet]]

3. Confirm the Structure of Your Schema

Make sure that both fields (iscountryCode and zip) are siblings in the object shape. This allows Yup to evaluate the relationship correctly and apply conditions based on the value of iscountryCode.

Conclusion

When using Yup for form validation, ensuring proper field referencing with string literals in the .when() condition is crucial. By following the steps above, you can alleviate the issue of encountering undefined results. This will let you build effective validation logic, particularly important in complex forms within JavaScript and React applications.

If you have further questions or need more assistance, feel free to reach out! Happy coding!