Getting a reference to an array element

Question

While I realize that an array, as a non-primitive data type, is handled by references in JavaScript, not by value, any particular element of that array could be a primitive data type, and I assume then that it is not assigned by reference.

I'd like to know how to get a reference to an individual element in an array so that I don't have to keep referring to the array and the index number while changing that element?

i.e.

var myElement=someArray[4]
myElement=5
//now someArray[4]=5

Am I misinterpreting various docs that imply but do not explicitly state that this is not the intended behavior?

you can use a getter/setter to turn just about anything into a "var" — dandavis
– dandavis, Commented Nov 7, 2013 at 1:01

Pointy · Accepted Answer · 2013-11-07 00:56:19Z

2

You can make a copy of an array element, but you can't create a value that serves as an alias for an array property reference. That's also true for object properties; of course, array element references are object property references.

The closest you could get would be to create an object with a setter that used code to update your array. That would look something like:

 var someArray = [ ... whatever ... ];

 var obj = {
   set element5(value) {
     someArray[5] = value;
   }
 };

Then:

 obj.element5 = 20;

would update someArray[5]. That is clearly not really an improvement over someArray[5] = 20.

edit — Now, note that if your array element is an object, then making a copy of the element means making a copy of the reference to the object. Thus:

var someArray = [ { foo: "hello world" } ];

var ref = someArray[0];

Then:

ref.foo = "Goodbye, cruel world!";

will update the "foo" property of the object referenced by someArray[0].

edited Nov 7, 2013 at 0:56

answered Nov 7, 2013 at 0:49

Pointy

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3 Comments

johnbakers Over a year ago

So anytime I want to change an element of an array, I must access that element in full, explicitly, using the index syntax. It seems wasteful, for example, if you want to do someArray[4].prop=4; someArray[4].foo=6 etc, you must index into the array every time, and if you do a lot of these in succession the program must look up the index every time

Pointy Over a year ago

If the array element is an object, then making a copy of it will save you that trouble. I'll extend the answer.

Mingle Over a year ago

Since you're trying to set a property on someArray[4] it seems like that is a non-primitive. In that case you can save off a reference to someArray[4] and use that instead. But as far as saving off a reference to a primitive, you can't do it without some form of boxing like the above answers.

John Foley · Accepted Answer · 2013-11-07 00:52:29Z

1

You can always pass around a closure to update this:

var myUpdater = function(x) {
  someArray[4] = x;
}
myUpdater(5);

If you want read/write capabilities, box it:

var makeBox = function(arr, n) {
  return {
    read: function() { return arr[n]; },
    write: function(x) { arr[n] = x; }
  };
} 

// and then:
var ptr = makeBox(someArray, 4);
ptr.read(); // original
ptr.write(newValue);
someArray[4]; // newValue

answered Nov 7, 2013 at 0:52

John Foley

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