How to copy array behaviour?

Question

I know that an array in JavaScript is nothing else than an object. When I define an array like that:

var array;
array = [ "a", "b", "c" ];

and run

Object.keys(array);

I get following array: ["0", "1", "2"]. Array length of array is 3.

When I add a property like:

array["a"] = "d";

Object.keys() is returning ["0", "1", "2", "a"], but array length of array is still 3.

But when I add a property like that:

array["3"] = "d";

the length of array is now 4.

If array is just another object, how can I achieve that kind of behaviour when I start my object from scratch like var myArray = {}?

Read this classic blog post from Kangax - it turns out that you cannot do what you're asking. — Pointy
– Pointy, Commented Aug 23, 2012 at 12:29
possible duplicate of Javascript array length incorrect on array of objects — epascarello
– epascarello, Commented Aug 23, 2012 at 12:29

nnnnnn · Accepted Answer · 2012-08-23 13:00:52Z

1

The .length property only includes properties with numeric indices, specifically those with integer values greater than or equal to zero.

If you're asking how to get a total count of all keys from an array or an object then you could do:

Object.keys(array).length

...since Object.keys() returns an array that will itself have a .length property.

answered Aug 23, 2012 at 13:00

nnnnnn

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2 Comments

nnnnnn Over a year ago

P.S. Note that .length could actually be greater than Object.keys(array).length because (as pointed out by Clyde) the length is one greater than the highest integer index, but JS allows sparse arrays and Object.keys() only returns indexes that have actually been assigned.

Amberlamps Over a year ago

So I am guessing that the value of length is calculated every time a new property is added to the object?

Clyde Lobo · Accepted Answer · 2012-08-23 13:05:00Z

1

The length property of array is the value of the highest numerical index + 1.

So after array["3"] = "d"; the highest numeric index is 3 hence the length returns 4

Object.keys(array).length should give you the length.

answered Aug 23, 2012 at 13:05

Clyde Lobo

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4 Comments

Amberlamps Over a year ago

array[1000000000] = "x" does not operate that way. Where is the limit?

Clyde Lobo Over a year ago

Try var array; array = [ "a", "b", "c" ]; array[1000000000] = 1; array.length;

Amberlamps Over a year ago

Add another 0 to the index and try again please.

Clyde Lobo Over a year ago

@Amberlamps : This is the reason why : stackoverflow.com/questions/6154989/…

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