Custom validation in Joi is not returning error message

2 min read 04-10-2024

Custom validation in Joi is not returning error message

Why Your Custom Joi Validation Isn't Throwing Errors: A Debugging Guide

Problem: You've implemented custom validation in your Joi schema, but it's not returning the expected error message. You're scratching your head, wondering why your validation logic isn't working.

Rephrased: You've created a custom rule in Joi to enforce a specific requirement in your data. However, even when your data violates this rule, Joi isn't throwing an error. You want to understand why this is happening and how to fix it.

Diving into the Code:

Let's imagine you have a schema to validate user input for a blog post:

const Joi = require('joi');

const blogPostSchema = Joi.object({
  title: Joi.string().required(),
  content: Joi.string().required(),
  category: Joi.string().valid('technology', 'lifestyle', 'travel'),
  author: Joi.string().custom((value, helpers) => {
    if (value.length < 3) {
      return helpers.error("string.min", { limit: 3 });
    }
    return value;
  })
});

// Example usage
const data = {
  title: 'My Blog Post',
  content: 'This is the content of the post.',
  category: 'technology',
  author: 'A' // Author name is too short
};

const { error } = blogPostSchema.validate(data);
console.log(error); // Output: null

In this example, we have a custom validation rule for the author field. It ensures that the author name is at least 3 characters long. However, even with the author field violating this rule, Joi.validate returns no error.

Unveiling the Mystery:

The culprit in this situation is the incorrect usage of the helpers.error function. It's designed to be used for throwing errors when a specific validation rule is violated. In this case, you are trying to use it for a custom validation rule, not an existing built-in rule.

Debugging and Solutions:

Understanding the helpers.error Function: The helpers.error function should be used to return errors based on pre-defined validation rules like string.min, string.max, etc. For custom validation rules, you should simply return a custom error message.

Correcting the Custom Validation: To address the issue, we should return a custom error message directly:

const blogPostSchema = Joi.object({
  // ... (rest of the schema)
  author: Joi.string().custom((value, helpers) => {
    if (value.length < 3) {
      return helpers.message("Author name must be at least 3 characters long."); 
    }
    return value;
  })
});

Leveraging Joi's Error Handling: You can further enhance the error message by using Joi's error handling mechanisms:

const blogPostSchema = Joi.object({
  // ... (rest of the schema)
  author: Joi.string().custom((value, helpers) => {
    if (value.length < 3) {
      return helpers.error("string.min", { limit: 3, message: "Author name must be at least 3 characters long." }); 
    }
    return value;
  })
});

Here, we've used helpers.error with a pre-defined error code (string.min) and a custom error message, providing a clear and structured error report.

Key Takeaways:

Use helpers.error only for pre-defined Joi validation rules.
For custom validation rules, return a custom error message directly or use helpers.error with a pre-defined error code and a custom message.
Use Joi's error handling features for clear and concise error reporting.

By understanding these principles and implementing them correctly, you can ensure your custom validation rules work seamlessly and provide informative error messages, making your code more robust and user-friendly.

References:

Joi Documentation: https://joi.dev/api/?v=17.7.0#custom-validation-rules
Error Messages in Joi: https://joi.dev/api/?v=17.7.0#error-messages