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JavaScript JavaScript Loops, Arrays and Objects Tracking Multiple Items with Arrays Removing Items From Arrays

Abhijit Das
Abhijit Das
5,022 Points

How to get previous object of the last item from the array list?

Hello friends, i want to retrieve the previous item from the last item of a arraylist. I tried the pop method but i am getting the last one, not the previous item of the last item. As an example if I add into my arraylist by input red,green,blue etc. After clicking the previous color btn I want green and after another click Red not blue first. How could i achive this. Pls help me.Thank you. https://codepen.io/pen/?editors=1010

3 Answers

Steven Parker
Steven Parker
210,423 Points

You can do that with the splice method:

In this case: "var secondlast = colors.splice(colors.length - 2, 1)[0];"

You could also create your own "pop2" method to work like "pop" but take the second-last item:

Array.prototype.pop2 = function () {
  if (this.length < 2) return;
  return this.splice(this.length - 2, 1)[0];
}

FYI: your link just goes to an empty codepen.

Aakash Srivastav
seal-mask
.a{fill-rule:evenodd;}techdegree
Aakash Srivastav
Full Stack JavaScript Techdegree Student 11,621 Points

Hey Steven , I didn't understand why you have put [0] at the end just before semicolon in your example. I have always used [0] only with "Array" name. But here how it is used and what it is doing?

"var secondlast = colors.splice(colors.length - 2, 1)[0];"

And my second question is -
"Is it necessary to write within this.splice() function?

  return this.splice(this.length - 2)[0];

Is it incorrect?Is this only return the second element and will not delete it?

Steven Parker
Steven Parker
210,423 Points

The "splice" method returns an array, and then the "[0]" gets the first element from it.

And "splice" does delete the selected element(s) from the array you use it on.

:point_right: Note: this comment has been updated for accuracy.

Aakash Srivastav
seal-mask
.a{fill-rule:evenodd;}techdegree
Aakash Srivastav
Full Stack JavaScript Techdegree Student 11,621 Points

Can we write pop function as-

var arr = [2, 4, 5, 6, 7];

function pop( accArray){
if(accArray.length > 2){
return this.splice(this.length - 2, 1)[0];
}
pop(arr);

Is it correct?

Steven Parker
Steven Parker
210,423 Points

If you want to create a function instead of an Array method, you'll need to replace "this" with your parameter name ("accArray"). Also, it looks like your function is missing the final closing brace (}). And you might want to allow it to work on arrays of exactly 2 items in addition to the longer ones.

And the name "pop" might be confusing — I called the Array method "pop2" to imply that it was removing the 2nd last item instead of the last.

Abhijit Das
Abhijit Das
5,022 Points

Thank you Steven Parker, you always help us whenever we stuck or lost in JavaScript Void..:) Thank you. "FYI: your link just goes to an empty codepen." - that's really a silly mistake from me. Sorry. Now i need another help. I want to get the next element. Suppose i add red,green,blue etc. color in my array list by using input.value. Once i click the previous color btn, getting now right element respectively such as green,red. Thanks for showing me the splice method(). Now once i click next color btn i want to get next color like respectively red,green etc. can you show me how can i get that result. I tried push method but the next btn doesn't showing any result. Thanks in advance. https://codepen.io/anon/pen/NgMOMX?editors=1010

Steven Parker
Steven Parker
210,423 Points

It's not clear to me what you want the "next color" button to do. But here's some observations:

  • "objCol" doesn't exist in the scope of the "next button" handler — did you mean to make it global?
  • you could use a plain "pop" instead of using slice with those arguments.
  • instead of doing "push" followed by "pop" you could skip both and use the variable directly
Abhijit Das
Abhijit Das
5,022 Points

Hi Steven,

I just want to create a color changing h1 element. My goal is simple, user can change header element color by input different color values and also can get previous or next color by clicking on previous and next color buttons. It's only for learning.

Steven Parker
Steven Parker
210,423 Points

What I mean is I'm not sure what the word "next" refers to. I thought the "next" one would be the one you enter and then hit "add".