监听/watch

<div id="app">
        {{total}}
    </div>
    <script>
        var vm = new Vue({
            el: "#app",
            data() {
                return {
                    firstName: 1,
                    lastName: 2,
                    total: 3
                }
            },
            /* 监听data中的值,只要值改变函数就会触发 */
            watch: {
                firstName() {
                    this.total = this.firstName + this.lastName
                },
                lastName() {
                    this.total = this.firstName + this.lastName
                }
            }
        })
    </script>