FormInputEl old value is assigned to wrong string

[edloidas]

When the oldValue is set, the string concatenation is used, that will automatically convert the right side value to string. It is probably not what was intended, as null will be converted to 'null' and undefined to 'undefined'.

this.oldValue = '' + value;

This convertion may lead to strange side effects, when the input was reset or is not yet dirty.