JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.aspnetcdn.com/ajax/knockout/knockout-2.2.1.js"></script>
present address <input data-bind="value: pratt"/><br>
<input type="checkbox" data-bind="checked: isPerm"/><br>
Permanent address <input data-bind="value: pmadd"/><br>
<input type="button" value="copy address" data-bind="click: copyToDefault" />

JavaScript

var vm = function() {
    var self = this;
    
    self.pratt = ko.observable('555');
    self.isPerm = ko.observable(false);
    self.pmadd = ko.observable('');
    
    self.copyToDefault = function() {
        self.pmadd(self.pratt());
        self.isPerm(true);
    }
    
    self.isPerm.subscribe(function(ch) {
    alert('1');
        if( !ch )
            self.pmadd('');
    });
}

var viewmodel = new vm();
ko.applyBindings(viewmodel);