How to detect a JavaScript array

"Piece of cafe" I bet you're thinking, oh I wish it were that easy.

But fear not, JavaScript does have the typeof operator (returns a string) which is very useful for type-checking:

var str = 'hello',
    num = 64,
    bool = false,
    func = function () {},
    undef;

typeof str; // "string"
typeof num; // "number"
typeof bool; // "boolean"
typeof func; // "function"
typeof undef; // "undefined"

Like shown in the previous code, it works fine for most things. However, it's a little flawed:

var n = null,
    str = new String('hello'), // any wrapper function or custom constructor
    arr = [1, 2, 3];

typeof n; // "object", which is wrong
typeof str; // "object", which is correct but not very useful.
typeof arr; // "object", which is correct but not very useful.

constructor property
Every created object has a constructor property that points to the function that created the object's prototype. You can leverage that reference to do some accurate detections:

var str = new String('hello'),
    arr = [1, 2, 3],
    obj,
    Person = function (name) {
        this.name = name;
    };

obj = new Person('John');

obj.constructor === Person; // true
str.constructor === String; // true
arr.constructor === Array; // true

isArray method
ECMAScript 5 defines a new method for the Array function called isArray(). If it's not implemented by the browser you can define it like this:

if (!Array.hasOwnProperty('isArray')) {
    Array.isArray = function (value) {
        return Object.prototype.toString.call(value) === '[object Array]';
    };
}

Invoking the toString() method of Object, gets us a string representation of the object, for arrays it's [object Array]:

var arr1 = [1, 2, 3],
    arr2 = new Array(1, 2, 3);

Array.isArray(arr1); // true
Array.isArray(arr2); // true
Array.isArray(new Number('12')); // false
Array.isArray({}); // false

Sources:
typeof operator (MDC)
constructor property (MDC)