What is the best way to count absolute array length in JavaScript?

Question

Just wondering, when you use array.length it gets the last index value and adds one. What if you have an array, that is defined this way for some reason:

    var myArray2 =[];
    myArray2[10]='x';
    myArray2[55]='x';

What is the absolute best way to get the true length of this Array? Something that would return 2 as the value.

I was thinking something like this, but not sure if there was already a method for this, or if there is a faster implementation.

Array.prototype.trueLength= function(){
    for(var i = 0,ctr=0,len=myArray2.length;i<len;i++){
        if(myArray2[i]!=undefined){
            ctr++;
        }
    }
    return ctr;        
}
console.log(myArray2.trueLength());

Zirak · Accepted Answer · 2012-09-27 12:20:29Z

9

Array.prototype.reduce only walks through existing indexes, so you can do:

var length = myArray2.reduce(function(sum) {
    return sum+1;
}, 0);

"But Uncle Zirak! reduce killed my parents!" Don't worry, young one, we can use Array.prototype.filter!

var length = myArray2.filter(function(item, idx) {
    return idx in myArray2;
}).length;

"All this array stuff is boring!" Well, whadya think of this!?

Object.keys(myArray2).length;

"But...but...but Zirak!!!! We're Amish, we don't have ECMAScript 5 yet!" Have no fear, Zirak is here!

for (var length = 0, i = 0; i < myArray2.length; i++) {
    if (i in myArray2) {
        length += 1;
    }
}

But at times like this, one has to wonder: Why do all that and defy the purpose of the array, a structured construct, instead of using something else fit for its purpose?

edited Sep 27, 2012 at 12:20

answered Sep 27, 2012 at 11:36

Zirak

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5 Comments

Tim Down Over a year ago

Nice. Of course, you'd need an implementation of Array.prototype.reduce for IE < 9

Zirak Over a year ago

@TimDown Compensated for that (and some more.)

Tim Down Over a year ago

But Uncle Zirak, I still have to work with old IE. Does that mean I'm Amish?

Zirak Over a year ago

@TimDown Just because all apples are fruit, doesn't make all that's a fruit an apple.

rlemon Over a year ago

Correct, some fruits can be lemons too!

Anoop · Accepted Answer · 2012-09-27 10:48:44Z

2

Iterate through array using for in. jsfiddle

Array.prototype.trueLength= function(){
  var ctr = 0;  
  for(var i in this){
        if(this.hasOwnProperty(i)){
            ctr++;
        }
    }
    return ctr;        
}
console.log(myArray2.trueLength());

answered Sep 27, 2012 at 10:48

Anoop

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Comments

yasaricli · Accepted Answer · 2012-09-27 11:03:45Z

1

Alternative Prototype method :

Array.prototype.trueLength= function(){
    var list= [], ctr = 0, array = this;

    for(var i in array) (function(arr) {

        if(array.hasOwnProperty(i)){
            list.push(arr);
            ctr++;
        };


    }(array[i]));

    return {length: ctr, "list": list}
}

sample:

var myArray2 =[];
    myArray2[10]='44';
    myArray2[55]='55';

// list not undefined 
myArray2.trueLength().list  // ["44", "55"]

// not undefined length list
myArray2.trueLength().length // 2

answered Sep 27, 2012 at 11:03

yasaricli

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Comments

Tim Down · Accepted Answer · 2012-09-27 11:30:50Z

0

I'd use the in operator:

function getArrayPropertyCount(arr) {
    var count = 0;
    for (var i = 0, len = arr.length; i < len; ++i) {
        if (i in arr) {
            ++count;
        }
    }
    return count;
}

answered Sep 27, 2012 at 11:30

Tim Down

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Comments

Powered · Accepted Answer · 2013-11-08 00:09:04Z

0

function TrueArrayLength (myArray2)
{
var TrueLength = 0;    
for ( var i = 0; i < myArray2.length; i++) 
{
    if (i in myArray2) 
    {
        TrueLength += 1;
    }
}

return TrueLength;
}

Use this function and get the true array lengths.

answered Nov 8, 2013 at 0:09

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