Edit in JSFiddle

var ViewModel = function (message) {

    this.message = ko.observable(message);
    this.message.subscribe(function(newValue){
        if(!this.couldSend())
            this.message(newValue.substr(0,140));
    }, this);
    
    this.remaining = ko.computed(function () {
        return 140 - this.message().length;
    }, this);
    
    this.couldSend = ko.computed(function () {
        return this.message().length > 0 && this.message().length < 140;
    }, this);
    
    this.show = function () {
        alert(this.message());
    };
    
};
ko.applyBindings(new ViewModel(""));

<h1>What's happening?</h1>
<textarea cols='50'  rows='3'  data-bind='value: message, valueUpdate:"afterkeydown"'></textarea>
<div class="remaining" data-bind='text: remaining'></div>
<br/>
<button data-bind='enable: couldSend'>Send</button>

* {
  font-family:Calibri;  
  margin: 5px
}

textarea {
    border: 1px solid rgba(200, 200, 200, 1);
    border-radius: 10px;
    padding: 4px;
}

.remaining {
    position: absolute;
    top: 57px;
    left: 341px;
    opacity: .3;
    font-weight: bold;
}