解决办法：
1、用ES6箭头函数，箭头方法可以和父方法共享变量
created () {
axios.get('http://localhost:3001/todos')
.then((res) => {
this.todos = res.data
})
.catch((error) => {
console.log(error)
})
}
2、在请求axios外面定义一下 var that=this
created () {
var that = this
axios.get('http://localhost:3001/todos')
.then(function (res) {
that.todos = res.data
})
.catch(function (error) {
console.log(error)
})
}
