javascript method array.shift() returning undefined

Question

I have this piece of code where I delete the first (or last) element from an array, according to a boolean variable. I do like this:

{...
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() 
}
}

The first log prints that waypoints has a certain length, then I call the method to delete elements and when the second log prints is length after undefined. "First" is a global variable initialized to true. Why is that?

As soon as your array is empty, you get undefined, as described here — lumio
– lumio, Commented Dec 26, 2017 at 10:28
Array.prototype.shift: "The shift() method removes the first element from an array and returns that removed element." — Andreas
– Andreas, Commented Dec 26, 2017 at 10:28
Update your return statement to return waypoints in deleteWaypoint — Hassan Imam
– Hassan Imam, Commented Dec 26, 2017 at 10:29
Also consider giving us more detail. How does your array look like? mcve — lumio
– lumio, Commented Dec 26, 2017 at 10:30
@Andreas Yep, modified now in waypoints.shift(); return waypoints. Thanks. Sorry for the silly question, if you answer I'll give you upvote and correct answer — Usr
– Usr, Commented Dec 26, 2017 at 10:30

user9141233 · Accepted Answer · 2017-12-26 10:40:06Z

2

change the function as below:

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)

answered Dec 26, 2017 at 10:40

user9141233

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Comments

JTeam · Accepted Answer · 2017-12-26 10:50:34Z

0

This works perfectly fine for me;

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

this.first = true
var waypoints = [1,2,3,4,5];
console.log("length before " + waypoints.length);
deleteWaypoint(waypoints)
console.log("length after " + waypoints.length);

Array.shift() returns the removed array, in below line copied from code posted in question,

waypoints = this.deleteWaypoint(waypoints)

the reference of array is getting re-assigned to the removed element which is not array and does not have length property and hence returning undefined

answered Dec 26, 2017 at 10:50

JTeam

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Comments

Rahul Patil · Accepted Answer · 2017-12-26 13:21:05Z

0

Yes, its correct because in return return waypoints.shift() will return single element which is deleted from array. You can do like this first, waypoints.shift() then, return waypoints

answered Dec 26, 2017 at 13:21

Rahul Patil

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