No. There are some cases where obj instanceof Array can be false, even if obj is an Array.

You do have to be careful with instanceof in some edge cases, though,
particularly if you're writing a library and so have less control /
knowledge of the environment in which it will be running. The issue is
that if you're working in a multiple-window environment (frames,
iframes), you might receive a Date d (for instance) from another
window, in which case d instanceof Date will be false — because d's
prototype is the Date.prototype in the other window, not the
Date.prototype in the window where your code is running. And in most
cases you don't care, you just want to know whether it has all the
Date stuff on it so you can use it.

Source: Nifty Snippets

This example, applies to array objects too and so on.

And the standard method suggest by ECMAScript standards to find the class of an Object is to use the toString method from Object.prototype and isArray(variable) uses it internally.