如何判断 javascript 变量是否为数组且仅是数组

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.

eg

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

Answer 2

I don't think that you can verify the integrity of something that was created as an Array object. Javascript won't prohibit you from adding properties to such an object.

You could iterate through the object's properties and return false if you see one that you don't think should be in Array , but that's still no guarantee that even the normal properties haven't had their values mangled.

Just don't add properties to Array s, then you won't have this problem. That you're asking about this kind of implies that you've done something quite wrong somewhere else in your code.

Answer 3

In general, you can't distinguish array and object. Object can be used as an array and vice versa. Example:

var fancyObj = {
    favoriteFood: "pizza",
    add: function(a, b){
        return a + b;
    }
};
fancyObj.add(2,3); // returns 5
fancyObj['add'](2,3); // ditto.