JavaScript type checks

JavaScript type checks have slight complexity. This and the reason that your code should be consistent through your project(s) make it necessary that type checks are part of your javascript programming guidelines.
I am using the recommodations from the jQuery project.

They say:

String

 typeof object === "string"  

Number

 typeof object === "number"  

Boolean

 typeof object === "boolean"  

Object

 typeof object === "object"  

Plain Object

 jQuery.isPlainObject(object)  

Function

 jQuery.isFunction(object)  

Array

 jQuery.isArray(object)  

Element

 object.nodeType  

null

 object === null

null or undefined

 object == null

undefined
     Global Variables

 typeof variable === "undefined"

     Local Variables

 variable === undefined

     Properties

 object.prop === undefined