Question

what’s the best way to determine whether a Javascript variable is an array, but has no other user-defined properties? ‘instanceof Array’ doesn’t do this.

e.g.

var var1 = [10,11];
var1['key1'] = 'extraProperty';

var1 instanceof Array;    //returns true

Answer 1

var isOnlyArray = function(o) {
    if (! (Object.prototype.toString.call(o) === "[object Array]")) {
        return false;
    }
    for (property in o) { 
        if (o.hasOwnProperty(property)) {
            var asInt = parseInt(property, 10);
            if (!(0 <= asInt && asInt < o.length)
                || String(asInt) !== property) {
                return false;
            }
        }
    }

    return true;
}

This function confirms that it’s an Array and that every defined property on an object is an integer index that’s in the range specified by .length.

var a = [1, 2];
console.log(isOnlyArray(a)); // true
a[2] = 4;
console.log(isOnlyArray(a)); // true
a["foo"] = 5;
console.log(isOnlyArray(a)); // false