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var viewModel = {
    message1: ko.observable(),
    message2: ko.observable()
};

ko.applyBindings(viewModel);

<h2>value binding</h2>

<input type="text" data-bind="value: message1" /> : <span data-bind="text: message1"></span>

<h2>textInput binding</h2>

<input type="text" data-bind="textInput: message2" /> : <span data-bind="text: message2"></span>