test.html

<!-- updating input will update automatically update span -->
<input type="text" data-bind="value: observable">
<span data-bind="text: observable"></span>

<!-- iterate over observableArray -->
<ul data-bind="foreach: observableArray">
  <li data-bind="text: $data"></li>
</ul>

<input type="text" data-bind="value: x">
<input type="text" data-bind="value: y">
<button data-bind="click: add">+</button>
<button data-bind="click: subtract">-</button>

<Calc params="x: myX, y: myY, result: myResult"></Calc>

test.js

ko.bindingHandlers.myValue = {
  init: function(element, valueAccessor) {
    if (!ko.isObservable(valueAccessor())) return;
    var observable = valueAccessor();
    function update() { observable(element.value); }
    element.addEventListener('change', update);
    ko.utils.domNodeDisposal.addDisposeCallback(element, function() {
      element.removeEventListener('change', update);
    });
  },
  update: function(element, valueAccessor) {
    element.value = ko.unwrap(valueAccessor()) || '';
  }
};


ko.bindingHandlers.yourBindingName = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called when the binding is first applied to an element
        // Set up any initial state, event handlers, etc. here
    },
    update: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called once when the binding is first applied to an element,
        // and again whenever any observables/computeds that are accessed change
        // Update the DOM element based on the supplied values here.
    }
};

function fcn(params) {
  function add() {
    return params.x + params.y;
  }
  return {
    x: params.x,
    y: params.y,
    add: add
  };
}

function fcn({ x, y }) {
  const add = () => x + y;
  return {x, y, add};
}

// create an observable with default value
var name = ko.observable('John');

// get the current value
name()  // returns 'John'

// set a new value
name('James')

// get the value again
name()  // returns 'James'