How to check that an variable is an array in JavaScript ?

Check that a variable is an Array:-

var data = [21,56,23,48,19], obj={};

Using a function:

function isArray(x) {

  return x.constructor.toString().indexOf("Array") > -1;

}

isArray(data);// true
Using  a method of ES 5: isArray() method of ES5 

returns true if an variable is an array else false.

var status= Array.isArray(data);

console.log(status) //true

var status= Array.isArray(obj);

console.log(status) //false

Using  a instanceof operator: instanceof operator 
returns true if an variable is created with a constructor else false.

data instanceof Array; // return true
obj instanceof Array; // return false