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K.Daube
Community Expert
K.DaubeCommunity Expert
Community Expert
June 15, 2016
Answered

Arrays are objects - really?

  • June 15, 2016
  • 1 reply
  • 1077 views

Dear experts and gurus,

The more I read the more I am confused. Every now and then the talk is "JS Arrays are objects and hence they are passed by reference". However, simple experiments leave my puzzled: It's the same as with light (depending on the view it behaves as waves or as particles):

gaSomething = ["a","b", "c"];
Update (gaSomething);
alert (gaSomething.join("\n"));         // the global array has changed
DeleteItem (gaSomething, "7");
alert (gaSomething.join("\n"));         // the global array has not changed
function Update (array) {
  for (var j= 0; j < 9; j++) {
    array.push(j+1); 
  }
}
function DeleteItem (array, item) {
  var index = IsInArray (array, item);
  var lower = array.slice (0, index);   // lower part
  var upper = array.slice (index+1);    // upper part
  var locArray    = lower.concat(upper);
  array = locArray;                     // transfer data
alert ("in DeleteItem:\n" + array.join("\n"));
}


The only safe assumption is that JS arrays are not treated as objects and hence are passed by value. This makes it difficult to set up functions to delete items from various arrays - for each array a distinct function is needed!

Any ideas for a general DeleteItem function ?

This topic has been closed for replies.
Correct answer Klaus Göbel

Ah, the length property is writable...

function DeleteItem (array, index) {
  delete array[index];                    // ...six,,three...
  array.sort();                            // six,three,two,
  array.length = array.length-1
}

This works as intended. Outside the function the global arry is now

eight,five,four,nine,one,seven,six,three,two

So "arrays are objects" is really true.

Thanks all for the discussion

Addendum:

If there is already an undefined item in the array, the final array will contain this at its last item.


Hi Klaus,

That is very dangerous !!!

1. array.length = array.length-1 means: delete the last item in an array

array.length = array.length-5 deletes the last 5 items in an array

delete array[index] only deletes the VALUE of the item !!!!!

When you sort it, it's at the end of the array and so you succeeded to delete that.

My code in my last post IS WRONG:

It must be :

  1. for (var j=9 ; j > 5; j --) { 
  2.     array.slice(j,1); 
  3.   }

However, I want to delete an item within the array. Unfortunately the splice function does not exist in ES...

It is slice - not splice !!

You really have to delete Items reverse.

Just try this function. It deletes items 3 and 2

function DeleteItem (array, index)
{
 $.writeln(array);
 for (var j=3 ; j > 1; j --)
    { 
    $.writeln("delete: " + array);
    array.splice(j,1);  
    $.writeln(array);
    }  
   
}

1 reply

Klaus Göbel
Klaus Göbel
Legend
June 15, 2016

Hi Klaus,

to delete several items in an array, you have to go backward:

  1. for (var j=9 ; j > 5; j --) { 
  2.     array.slice(j);  
  3.   } 

In an arry with 10 items: if you delete item 9 the array now has 9 items. Item 10 doesn't exist any more.It is now item 9

If  you delete item 9 NEXT you delete the former item 10 (the last original item).

K.Daube
Community Expert
K.DaubeCommunity ExpertAuthor
Community Expert
June 16, 2016

Thanks Klaus, for Your answer.

However, I want to delete an item within the array. Unfortunately the splice function does not exist in ES...

Looking at my example script:

  • At line 05 gaSomething is a,b,c,1,2,3,4,5,6,7,8,9
  • Lines 16 and 17 do not change argument array: a,b,c,1,2,3,4,5,6,7,8,9
  • At line 19 array is a,b,c,1,2,3,4,5,6,8,9 - the item with value 7 has been removed.
  • But this is all within the function - obviously not on the global array gaSomething
  • which is after the function call: a,b,c,1,2,3,4,5,6,7,8,9

Again I read  (O'Reilly: JavaScript: The Good Parts, 2013 - BTW written with FM 5.5.6)

that «In JavaScript, arrays are objects, functions are objects, regular expressions are objects, and, of course, objects are objects.»
Hence I tried this:

gaSomething = ["one","two", "three", "four", "five", "six", "seven", "eight", "nine", "ten"];
gaSomething.sort();
Console (gaSomething);                  // eight,five,four,nine,one,seven,six,ten,three,two
DeleteItem3 (gaSomething, 7);
Console (gaSomething);                  // eight,five,four,nine,one,seven,six,three,two,
function DeleteItem2 (array, index) {
  var locArray = array;
  delete locArray[index];                    // ...six,,three...
  locArray.sort();                            // six,three,two,
  array= locArray.slice(0,locArray.length-1); // six,three,two <- no last undefined
Console ("array within function\n" + array);
}
function DeleteItem3 (array, index) {
  delete array[index];                    // ...six,,three...
  array.sort();                            // six,three,two,
  array= array.slice(0,array.length-1);    // six,three,two <- no last undefined
Console ("array within function\n" + array);
}

Whether I use DelteItem2 or ...3 in line 05, the results are the same: outside of the function the global array still has it's last undefined item (indicated by the last comma in the console log - or alert).
How to get rid of this?

K.Daube
Community Expert
K.DaubeCommunity ExpertAuthor
Community Expert
June 16, 2016

Ah, the length property is writable...

function DeleteItem (array, index) {
  delete array[index];                    // ...six,,three...
  array.sort();                            // six,three,two,
  array.length = array.length-1
}

This works as intended. Outside the function the global arry is now

eight,five,four,nine,one,seven,six,three,two

So "arrays are objects" is really true.

Thanks all for the discussion

Addendum:

If there is already an undefined item in the array, the final array will contain this at its last item.

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