JSFiddle - React, Tailwind, and code Playground

HTML

<h1 data-bind="text: sample_data()[0][1][0].title1"></h1>

JavaScript

var self = {};
self.sample_data = ko.observableArray([
  {
  "1":[
        { title1:"abc11", title2:"def11"},
        { title1:"abc12", title2:"def12"}
    ],
  "2":[
        { title1:"ghi21", title2:"jkl21"},
    ]
   }]);  
      
ko.applyBindings(self);