how to get length of javascript "array"

Question

We have been using javascript "hashes" a lot lately, and we've been looking for a universal way to count the items contained in both arrays and hashes without having to "know" which we're dealing with except in the count method. As everyone knows .length is useless since it only returns the value of the highest index in the array. What we have below does not work because hashes test true for Array, but the length value returned is crap for hashes. We originally replaced .length all over our project with Object.keys().length, but this isn't supported in IE8 and lower.

This is such a stupid simple thing and we can't seem to get it working. Help me, Obi Wan. You're my only hope!

function isNullOrUndefined(aObject) {
    "use strict";
    return (typeof aObject === 'undefined' || aObject === null);
}

function count(aList) {
    "use strict";
    var lKey = null,
        lResult = 0;
    if (!isNullOrUndefined(aList)) {
        if (aList.constructor == Array) {
            lResult = aList.length;
        } else if (!isNullOrUndefined(Object.keys)) {
            lResult = Object.keys(aList).length;
        } else {
            for (lKey in aList) {
                if (aList.hasOwnProperty(lKey)) {
                    lResult++;
                }
            }
        }
    }
    return lResult;
}

is the hash only a literal object?

elmuchacho
– elmuchacho

2012-04-05 16:17:39 +00:00
Commented Apr 5, 2012 at 16:17 — elmuchacho
– elmuchacho, Commented Apr 5, 2012 at 16:17

Raynos · Accepted Answer · 2012-04-05 16:17:25Z

3

Object.keys polyfill copied verbatim from the ES5-shim

// ES5 15.2.3.14
// http://es5.github.com/#x15.2.3.14
if (!Object.keys) {
    // http://whattheheadsaid.com/2010/10/a-safer-object-keys-compatibility-implementation
    var hasDontEnumBug = true,
        dontEnums = [
            "toString",
            "toLocaleString",
            "valueOf",
            "hasOwnProperty",
            "isPrototypeOf",
            "propertyIsEnumerable",
            "constructor"
        ],
        dontEnumsLength = dontEnums.length;

    for (var key in {"toString": null}) {
        hasDontEnumBug = false;
    }

    Object.keys = function keys(object) {

        if ((typeof object != "object" && typeof object != "function") || object === null) {
            throw new TypeError("Object.keys called on a non-object");
        }

        var keys = [];
        for (var name in object) {
            if (owns(object, name)) {
                keys.push(name);
            }
        }

        if (hasDontEnumBug) {
            for (var i = 0, ii = dontEnumsLength; i < ii; i++) {
                var dontEnum = dontEnums[i];
                if (owns(object, dontEnum)) {
                    keys.push(dontEnum);
                }
            }
        }
        return keys;
    };

}

answered Apr 5, 2012 at 16:17

Raynos

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2 Comments

Alex Turpin Over a year ago

If it wasn't clear to OP: This is a shim to make Object.keys work in IE8.

Mark Lauter Over a year ago

Raynos, I gave you an upvote because - hazah! that's what we need today! but I'm going to give answer to elmuchacho because that's what we need long term. thank you very much!

elmuchacho · Accepted Answer · 2012-04-05 16:27:43Z

1

Despise the answer from Raynos that is completely valid please consider performance

This is how my hash object look like

function Hash(){
   this.values = [];
   this.keys = {};
}
Hash.prototype.set = function(key, val){ 
   if(this.keys[key]){
      this.values[this.keys[key]] = value
   }else{
      this.keys[key] = (this.values.push(val)-1)
   }
}
Hash.prototype.length = function(){
    return this.values.length
}

Why I do this is simple performance looping through an object to count the properties length will be really inefficient the solution above give you direct access all the time.

answered Apr 5, 2012 at 16:27

elmuchacho

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2 Comments

Raynos Over a year ago

Sure your length operator is faster, but your memory usage increased and your set operator is significantly slower. You need real benchmarking to figure out where your bottlenecks are. This also hurts readability and introduces unncessary complexity. Object.keys isn't that slow

Mark Lauter Over a year ago

I agree with your comment about benchmarking. We don't have to get the count very often, so Object.keys is good for what we're doing. However, the hash idea above is quite good for certain applications.

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