javascript - Type checking for array

When I do: typeof([]) it returns object.

I've heard its not really acceptable to edit the prototypes of JavaScript's inbuilt types.

However, if I do this: Array.prototype.isArray = true; and then the following work:

var arr = [];
var obj = {};

if (arr.isArray)
{
    console.log("Array");
}
else
{
    console.log("Not array");

}

if (obj.isArray) // undefined
{
    console.log("Array");
}
else
{
    console.log("Not array");
}

Or is this still not acceptable? What would be a better way around this?