popup.js

var STORE_KEY = 'jsoneditor-text';
var editor = null;

function loadEditor() {
  var container = document.getElementById('jsoneditor');
  var options = { mode: 'code', onChange: onChange };
  editor = new JSONEditor(container, options);
  editor.setText(localStorage.getItem(STORE_KEY) || JSON.stringify({}));
}

function onChange() {
  localStorage.setItem(STORE_KEY, editor.getText());
}

document.addEventListener('DOMContentLoaded', loadEditor, false);