Async Validation

If you have wondered how async validation works with Strickland, you will be delighted at how simple it is: a validator can use a Promise.

Similar to how validate normalizes a boolean into validation result with an isValid property, a Promise is normalized into a validation result with a validateAsync property. The normalized validateAsync property is a function that returns a Promise resolving to the async result.

Let's take a look at a usernameIsAvailable validator. Just like synchronous validators, async validators can resolve to either boolean results or validation results with isValid properties.

import validate from 'strickland';

function usernameIsAvailable(username) {
    return new Promise((resolve) => {
        if (username === 'marty') {
            // Resolve to an invalid validation result object
                isValid: false,
                message: `"${username}" is not available`

        // Resolve to a boolean

const result = validate(usernameIsAvailable, 'marty');

result.validateAsync().then((asyncResult) => {
    asyncResult = {
        isValid: false,
        value: 'marty',
        message: '"marty" is not available'

When a validator returns a Promise, the normalized validation result will include isValid: false to indicate that the result is not (yet) valid.

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