Validation Result Props

You may have noticed that examples so far have not included any validation messages. Validation messages are often the heart of validation libraries, but Strickland has no awareness of them. This ensures you can build your application with the user experience you need, including tone and localization of your own messages, without any interference from Strickland or any possibility of a default validation message leaking through.

But how do you get validation messages on results? As was mentioned in our core concepts, a validator can return either a boolean or a validation result object with an isValid property. Validation result objects can have additional properties; those properties flow through Strickland's validate function to be available to your application.

Let's extend the letter validator to include a message prop on its result.

import validate from 'strickland';

function letterValidator(validatorProps) {
    return function validateLetter(value, context) {
        // Be sure not to overwrite the original
        // validatorProps variable
        let resolvedProps = validatorProps;

        if (typeof resolvedProps === 'function') {
            resolvedProps = resolvedProps(context);
        }

        resolvedProps = resolvedProps || {};

        const {letter} = resolvedProps;

        return {
            isValid: (value === letter),
            message: `Must match "${letter}"`
        };
    }
}

const validator = letterValidator({letter: 'B'});
const result = validate(validator, 'A');

/*
    result = {
        isValid: false,
        message: 'Must match "B"',
        value: 'A'
    }
 */

Validation results can include any additional properties that are needed for your application regardless of whether the result is valid. Your application can then decide how and when to consume those properties. As an example, some user interfaces show successful validation messages to give positive feedback to the user.

Strickland will ensure that all validation results have an isValid property that is a boolean, and Strickland will always add a value property that reflects the validated value, but the rest is up to you. Validation messages are perhaps the most common property added, but validators can include any details needed on the results.