obj.length === +obj.length

length.js

var test = function(obj) {
  console.log(obj);
  console.log(obj.length);
  console.log(+obj.length);

  if (obj.length === +obj.length) {
    console.log("array!!!");
  } else {
    console.log("object!!!");
  }
};

test([1,2,3,4,5]);
test({a:1,b:2,c:3});

/* node length.js
[ 1, 2, 3, 4, 5 ]
5
5
array!!!
{ a: 1, b: 2, c: 3 }
undefined
NaN
object!!!
*/

obj maybe not a Object. maybe arguments. maybe var obj = {length: 5}