Validate Input Field if float number is less 0 , display message without any external library

Hi Team
First of all, love Vue.js started using it and I just love it. I have got up to a point where I am dynamically using computed property for input field. Is there any way I can validate the input field is value entered less than $0 without using external library. I can only use script tab not the library itself due to system limitations. Also, if someone enters the value less than zero is there any way I can display alert or a model message.

Thanks a lot in advance