<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>Vue.js - Florian Studio</title> </head> <style> .class1{ background: #444; color: #eee; } </style> <body> <script src="https://cdn.staticfile.org/vue/2.4.2/vue.min.js"></script> <div id = "app"> <p style = "font-size:25px;">カウンター: {{ products.id }}</p> <button @click = "products.id++" style = "font-size:25px;">ここにクリックしてください</button> </div> <script type = "text/javascript"> var myproduct = {"id":1, name:"book", "price":"20.00"}; var vm = new Vue({ el: '#app', data: { products: myproduct } }); Vue.set(myproduct, 'qty', 1); console.log(vm); vm.$watch('products.id', function(nval, oval) { alert('変更前の値：' + oval + ' 変更後の値: ' + nval + '!'); }); </script> </body>