JavaScript Array undefined element

Question

I´ve got a function which should add an element at the start of an array. But I always get an undefined element at the end of my array. I hope someone can help me :)

function putToFirst(e){
   var array = [];
   array.push(e);
   this.arrayList = array.concat(this.arrayList);
}

EDIT:

class List {

  constructor () {
    super()
    this.arrayList = [];
  }

  putToFirst(e) {
      this.ArrayList.unshift(e);
 }
}

thats the class. I create a new object from the class list and call the function putToFirst on this object. But I always get an Array with 'undefinded' in the end

Note that if you're going with `unshift` on `this.arrayList`, you'll mutate the array instead of replacing it with a new one. — user3297291, Dec 09 '16 at 15:25
e is an integer which should be added in to the array. this is the object on which the function is called — Alex9677, Dec 09 '16 at 15:25
@Alex9677 — And what is that object? How are you testing the results? Provide a [mcve] — Quentin, Dec 09 '16 at 15:33
So, is it `this.ArrayList` or `this.arrayList`? They’re not the same thing. — Sebastian Simon, Dec 09 '16 at 17:51
Why do you invoke the mother constructor **super()** ? Does your Class inherit from a mother Class — kevin ternet, Dec 10 '16 at 18:26
You declared the property arrayList of your class in the constructor but after you call this.ArrayList. That can't work — kevin ternet, Dec 11 '16 at 09:06

score -4 · Answer 1 · answered Dec 09 '16 at 15:28

-4

this.arraylist is wrong this represent a current object you are not currently using any class in above code you just need to change 

  function putToFirst(e){
   var array = [];
   var arrayList=[];
   array.push(e);
  arrayList = array.concat(arrayList);
  console.log(arrayList);
}

answered Dec 09 '16 at 15:28

hithin chandran

43
3

I wouldn’t assume that `this.arrayList` is wrong. However, your function creates a new and empty `arrayList` every time. This has nothing to do with appending new elements to the beginning of the array. – Sebastian Simon Dec 09 '16 at 15:33
thanks for showing my mistak – hithin chandran Dec 10 '16 at 07:19

JavaScript Array undefined element

1 Answers1