How to access field in custom widget?

I'm building a custom widget that I want to render differently depending on if the field data is valid or not.
How do I check the valid state from a widget?

```
var React = require('react'),
Base = require('newforms').TextInput;

var UnderlineInput = Base.extend({
constructor: function UnderlineInput(kwargs) {
if(!(this instanceof UnderlineInput)) {
return new UnderlineInput(kwargs);
}

Base.call(this, kwargs);
}
});

UnderlineInput.prototype.render = function(name, value, kwargs) {
var finalAttrs = this.buildAttrs(kwargs.attrs, {
name: name
});

var valueAttr = kwargs.controlled ? 'value' : 'defaultValue';
finalAttrs[valueAttr] = value;

return (

module.exports = UnderlineInput;
```