Implementing an Array.count() Method in JavaScript

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As much as I write about MooTools on my blog, I spend most of my work day knee-deep in PHP. As you probably know, one way to get the size of an array in PHP is to use the count() function:

echo count($my_array);

In JavaScript, the way to get the size of an array is to use the length property, like this:

alert(my_array.length);

For some reason, I absolutely hate the ".length" way of retrieving the length of an array. For this reason, I've implement the count() method into JavaScript:

Array.prototype.count = function() {
	return this.length;
};

The count() habit is difficult to break, so why try?

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Discussion

  1. Thanks for your blog. you offer great advice and tutorials and for some odd reason, if I am looking to see how to complete a task, you have a tutorial on it or at least an idea on the concept I am trying to achieve; so thanks David. Although this post came about 2 days too late. :)

  2. Do you do this for strings as well?

  3. @Binny: No, I don’t. Just arrays.

  4. Jason

    This is all fine and well, but length is fundamentally different from count.

    Take the following example:

    var myArray = [‘one’, ‘two’, ‘three’];
    // myArray.length == 3, as we would expect

    var myOtherArray[100] = ‘one’;
    //myArray.length == 100, NOT 1

    Even though we only have 1 item in the array, ‘length’ refers to the largest index + 1. Unlike PHP where we can use numeric keys and strings for indexes in an ad hoc way and still get an accurate count via count(), in JS it’s not quite the same.

    In a way, this is just splitting hairs, but a lot of people already don’t understand JS’s length property, and referring to it as “count” only serves to further the confusion.

  5. Jason

    Correction to my previous post:

    the second code snippet would be:

    var myOtherArray = [];
    myOtherArray[100] = ‘one’;

    and its length property would actually return 101.

  6. riv

    That’s right and that’s what I was expecting when searching for count on js. This should do it:

    function count(array)
    {
        var c = 0;
        for(i in array) // in returns key, not object
            if(array[i] != undefined)
                c++;
    
    return c;
    }
    
    • Riv, thats a much better form of the count use. Its surprising that this is not built into javascript.

  7. Christian
    Array.prototype.count = function () {
        for (k in this) { if (this[k].constructor !== Function) { this.length++; } };
        return this.length;
    }
    
  8. I think it’s okay for personal project but it has a counter side when you are in enterprise. The maintainability is harder when people use custom extension (prototype existing framework object). It cane be confusing. In some case, I have seen people creating function to simply get an object inside an array. I think this should be used with parsimony. But, it’s just my opinion…

  9. Iskren Antonov
    Array.prototype.count = function () {
    	var counter = 0; // Initializing main counter
    	for(i in this) // Looping through elements
    		if(typeof this[i] != "undefined") // If empty it's undefined
    			counter++; // Counting not empty elements
    	return counter-1; // Excepting own function
    }
    

    This is a working one. It counts function elements too.

  10. Lineo

    I’ve just bumped into learning Js, I try to be as attentive as possible but, I don’t find a way. So, may any one help me how to understand Js programming.

  11. Rowan Adinghey
    Array.prototype.count = function (a) {
    	var c = 0;
    	for(var i = 0; i < this.length; i++) {
    		if(typeof(this[i]) !== "undefined") {
    			if(typeof(this[i]) == "string" && this[i].length == 0) {
    			} else { 
    				c++;
    			}
    		}
    	}
    	return c;
    }
    

    OR

    
    count = function (a) {
    	var c = 0;
    	for(var i = 0; i < this.length; i++) {
    		if(typeof(a[i]) !== "undefined") {
    			if(typeof(a[i]) == "string" && a[i].length == 0) {
    			} else { 
    				c++;
    			}
    		}
    	}
    	return c;
    }
    

    Wow, no functionality to edit comments? What is this, the year 2001?

  12. Querying the length is faster than invoking a method, but the habit of using .count() is interesting.
    In python is used len([1, 2, 3]) function to get the count, so the JavaScript’s length is not so bad :).

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