Because validate returns synchronously, your application must recognize when async validation needs to be resolved. The validation result returned from validate will only include the validateAsync property when a async validation needs to be resolved. If the validateAsync result property exists, it will be a function that returns a Promise.

const result = validate(usernameIsAvailable, 'marty');
​
// The application defines a `handleValidationResult` function
// to handle validation results when the are completed
​
if (result.validateAsync) {
    result.validateAsync().then((asyncResult) => handleValidationResult(asyncResult));
} else {
    handleValidationResult(result);
}

Async Validation Helper: validateAsync

To avoid boilerplate code, Strickland provides a validateAsync function as a named export. This function always returns a Promise, regardless of whether validation completed synchronously or needs to be resolved asynchronously.

import {validateAsync} from 'strickland';
​
const result = validateAsync(usernameIsAvailable, 'marty');
result.then((asyncResult) => handleValidationResult(asyncResult));