watch
Type: Object
Details: An object where keys are expressions to watch and values are the corresponding callbacks. The value can also be a string of a method name, or an Object that contains additional options. The Vue instance will call $watch() for each entry in the object at instantiation.
Example: var vm = new Vue({
data: {
a: 1
},
watch: {
'a': function (val, oldVal) {
console.log('new: %s, old: %s', val, oldVal)
},
// string method name
'b': 'someMe