What is the method for determining the length of a JavaScript "array"?

Question

What is the method for determining the length of a JavaScript "array"?

Lately, we've been heavily using javascript arrays and hashes and trying to find a universal way to count the items in both without needing to differentiate between the two. The .length method has proved to be unreliable as it only returns the value of the highest index in an array. The code snippet below attempts to solve this issue, but runs into problems with hashes returning inaccurate length values. We initially switched to Object.keys().length, but faced compatibility issues with older browsers like IE8.

We're stuck on such a simple problem and need some guidance. Help me, Obi Wan Kenobi. 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;
}

javascript arrays hash count

Answer 1

Answer №1

Object.keys polyfill taken directly from the ES5-shim repository

// Custom implementation of ES5 15.2.3.14
// Source: http://es5.github.com/#x15.2.3.14
if (!Object.keys) {
    // Reference for safer Object.keys compatibility
    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;
    };

}

Answer 2

Object.keys polyfill taken directly from the ES5-shim repository

// Custom implementation of ES5 15.2.3.14
// Source: http://es5.github.com/#x15.2.3.14
if (!Object.keys) {
    // Reference for safer Object.keys compatibility
    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;
    };

}

Answer 3

Answer №2

Although Raynos may have provided a valid answer, it is important to consider performance.

Here is an example of my hash object:

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
}

The reason I implement this structure is for performance reasons. Iterating through an object to determine its length can be inefficient, whereas the solution above provides direct access at all times.

Answer 4

Although Raynos may have provided a valid answer, it is important to consider performance.

Here is an example of my hash object:

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
}

The reason I implement this structure is for performance reasons. Iterating through an object to determine its length can be inefficient, whereas the solution above provides direct access at all times.

What is the method for determining the length of a JavaScript "array"?

Answer №1

Answer №2

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