Object.prototype.toString( )

When the toString method is called, the following steps are taken:
1. Get the [[Class]] property of this object.
2. Compute a string value by concatenating the three strings “[object “, Result (1), and “]”.
3. Return Result (2)

The [[Class]] property of the newly constructed object is set to “Array”.

function isArray(o) {   return Object.prototype.toString.call(o) === '[object Array]';  } 

var arr = []; return arr instanceof Array;