knockout.js - Unable to parse bindings. knockout error -

this day 2 learning knockout.

trying attach "deleteitem" button click. gives following error.

error

uncaught error: unable parse bindings.
message: referenceerror: deleteitem not defined; bindings value: click: deleteitem

javascript:

$(function () {     var defaultdata = [{         id: 1,         item: "todo 1"     }, {         id: 2,         item: "todo 2"     }, {         id: 3,         item: "todo 3"     }];     var viewmodel = {         listitem: ko.observablearray(defaultdata),         additem: function () {             // add new item             var id = this.listitem().length + 1;             this.listitem.push({                 id: id,                 item: "todo " + id             });         },         deleteitem: function () {             alert(this);         }     }     ko.applybindings(viewmodel, main); }); 

html:

<div id="main">     <button data-bind="click: additem">+ add item</button>     <div data-bind="foreach: listitem">         <input type="text" data-bind="value: item" />         <input type="button" data-bind="click: deleteitem" />         <br />     </div> </div> 

the function deleteitem on view model. when you're binding inside foreach, context of binding operation individual item listitem array. need bind $root.deleteitem reference root view model.