vue里面这个foreach中this为什么指向undefine?

Question

 var a = [{name: 'yang',age:1},{school:'one'}]
console.log(this)  //window
this.a.forEach(function () {
  console.log(this) //window
})

forEach是不是不改变this指向，为什么下面这个图里面forEach中this由组件变成了undfine?

https://gitee.com/qflyalltime/travel/blob/master/src/pages/city/components/Search.vue
第55行

Answer 1

打印的是window啊。。没问题。。

 <script src="https://cn.vuejs.org/js/vue.js"></script>
<div id="watch-example">
    <p>
        Ask a yes/no question:
        <input v-model="keyword">
    </p>
</div>
<script>
    var watchExampleVM = new Vue({
        el: '#watch-example',
        data: { cities: [[1, 2], [3, 4]] ,keyword:null},
        watch: {
            keyword: function (newValue, oldValue) {
                setTimeout(()=>{
                    console.log(this)
                    for (let i in this.cities) {
                        this.cities[i].forEach(function (item) {
                            console.log(item,this)
                        });
                    }
                },1000)
            }
        }
    });
  </script>