({
    register : function(component, event) {
        var inputCmp = component.find("inputCmp");
        var value = inputCmp.get("v.value");
        // is input valid text?
        if (value === "John Doe") {
            inputCmp.setCustomValidity("John Doe is already registered");
        } else {
            inputCmp.setCustomValidity(""); // if there was a custom error before, reset it
        }
        inputCmp.reportValidity(); // Tells lightning:input to show the error right away without needing interaction
    }
})