Find Array length in Javascript

Question

This will sound a very silly question. How can we find the exact length of array in JavaScript; more precisely i want to find the total positions occupied in the array.

I have a simple scenario which i guess most of you might be aware of.

var a = [1,2,3];
a.length; //This will output 3

Now if i give

a[100] = 100;
a.length; // The output is 101; 

I want to get the exact size of the array which in the above case should be 4.

Answer 1

What you are looking for is not the length of an array but the number values allocated in that array.

Array.length will NOT give you that result but the total number of values allocated.

A workarround is to count the properties of the object behind the array, with:

Object.keys(a).length

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Relationship_between_length_and_numerical_properties

But with some caveats:

• It will also count literal properties, like a.a_property. I do not think that is what you want. So, you will have to filter that result:

!(+el % 1) which check if el can be considered as numerical property even if it has a type of string.

• you want count only positive integers, so you have to filter them with:

+el>=0

• finally, as array size is limited to 2^32, you will to also filter positive integers greater than that:

+el < Math.pow(2,32)

Functionally, you will have your result with this filter:

Array.realLength= Object.keys(a).filter(function(el){return !(+el % 1) && +el>=0 && +el < Math.pow(2,32) ;}).length 

Answer 2

TL;DR The simplest reliable approach that I can think of is the following:

var count = a.filter(function() { return true; }).length;

In modern JavaScript engines, this could be shortened to:

var count = a.filter(() => true).length;

---

Full answer:

Checking against undefined isn't enough because the array could actually contain undefined values.

Reliable ways to find the number of elements are...

Use the in operator:

var count = 0;
for (var i = 0; i < a.length; i += 1) {
    if (i in a) {
        count += 1;
    }
}

use .forEach() (which basically uses in under the hood):

var a = [1, undefined, null, 7];
a[50] = undefined;
a[90] = 10;

var count = 0;
a.forEach(function () {
    count += 1;
});

console.log(count);    // 6

or use .filter() with a predicate that is always true:

var a = [1, undefined, null, 7];
a[50] = undefined;
a[90] = 10;

var count = a.filter(function () { return true; }).length;

console.log(count);    // 6