javascript - javascript方法array.shift()返回未定义

我有一段代码，根据布尔变量，从数组中删除了第一个（或最后一个）元素。我喜欢这样：

{...
console.log("length before " + waypoints.length)
waypoints = this.deleteWaypoint(waypoints)
console.log("length after " + waypoints.length)
...}

deleteWaypoint(waypoints){
if (this.first){
  this.first = false;
  return waypoints.shift()
} else {
  this.first = true;
  return waypoints.pop()
}
}

第一个日志打印的waypoints具有一定的长度，然后我调用删除元素的方法，第二个日志打印的是length after undefined。 “第一”是初始化为true的全局变量。
这是为什么？

最佳答案

更改功能如下：

var  waypoints = [1,2,3,4,5,6,7,8,9,0];

var deleteWaypoint = (waypoints)=>{
if (this.first){
  this.first = false;
  waypoints.shift();
  return waypoints
} else {
  this.first = true;
  waypoints.pop()
  return waypoints
}
}

console.log("length before " + waypoints.length)
waypoints = deleteWaypoint(waypoints)
console.log("length after " + waypoints.length)