You can use javascript to perform validation on a field. The way to respond is by setting the valid variable. If it is set to true then the validation passes. If you set it to a string, the validation fails and the validation message is set to whatever the valid variable is set to.
In addition, input variable is set to the value that has been entered in the field. The component variable is set to the definition of the field.
You can also reference other resources and properties for validation. For example, if there is a user resource with a password field, you can use its value with user.password
Below is an example of a custom field validation rule on a text area control to ensure that the user input is not longer than a certain length:
valid = (input.length < 250) ? true : "Maximum character length reached.";
