Get next array element

Question

Say I want to replace an array of objects with an array of decorators:

var arr = [{id:1, text: "text1"}, {id:2, text: "text2"}, {id:3, text: "text3"}]; 

arr.forEach(function(el, idx){
  var newEl = new NewEl(el.id, el.text, arr[idx + 1], idx); 
  arr[idx] = newEl; 
}, this); 

console.log(arr); 

function NewEl(id, text, nextEl, idx){
  this.id = id; 
  this.text = text; 
  this.next = nextEl; 
}

nextEl will still refer to the old object, not the next item in the array (whatever that element).
How do I pass a reference to the next element (location) in the array to the constructor function?

Note: If possible, I'd prefer not to use a workaround by modifying the logic within the loop (workaround involving setting 'next' on the previous element on next iteration).

Is it possible?

@Qwerty I need both prev/next. Should have mentioned that though. — Trace
– Trace, Commented May 15, 2016 at 4:25

Matt Way · Accepted Answer · 2016-05-15 04:00:08Z

4

You can't use a reference before that reference is created (i'm sure that's obvious). So it is difficult to solve without changing the loop to look at the previous. However, if you must solve it without looking at the previously created items, you could change your NewEl to receive both the full array and next index instead of an actual reference. Something like the following:

function NewEl(id, text, arr, nextIndex){
    this.id = id; 
    this.text = text; 
    this.arr = arr;
    this.nextIndex = nextIndex; 
}

NewEl.prototype.getNext = function(){
    return this.arr[this.nextIndex];
};

The idea being that the only way you can use a reference prior to creating it, is to actual not use it, but store a way of accessing it later. However this is a terrible way of doing this, as any change to the array will break the getNext() function.

tldr; You should really just change the loop to reference the previous item, from the next iteration.

answered May 15, 2016 at 4:00

Matt Way

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3 Comments

Trace Over a year ago

Jup a getter would work. I thought indd about storing the current index instead of next, then use index+1 in the getter (this wouldn't break?). I agree with you that using the loop works, but it means that on every insert / delete, I'll have to modify the previous and next nodes. Not a problem though, just didn't seem very clean at first thought.

Matt Way Over a year ago

When you are building linked graphs you usually need to update link siblings on graph change events. It is clean.

Trace Over a year ago

Ok, then this is how I'll do it. Thanks!

Redu · Accepted Answer · 2016-05-15 06:42:49Z

1

You may do like this too

var arr = [{id:1, text: "text1"}, {id:2, text: "text2"}, {id:3, text: "text3"}];
arr.reduceRight((p,c) => (c.next = p,c));
arr.reduce((p,c) => (c.prev = p,c));

Since in one comment you mention you would like previous as well i added prev property too.

answered May 15, 2016 at 6:42

Redu

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1 Comment

Trace Over a year ago

Yes, thanks for the suggestion. Since I have a tree structure with nested arrays of node objects I'll probably go the iterative way.

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