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
resolve({
isValid: false,
message: `"${username}" is not available`
});
}
// Resolve to a boolean
resolve(true);
});
}
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.