type

annotated.js

// var type = 

function(
  a                     // target
){
  return a == null ?    // if null or undefined
  a + '' :              // then convert it to string                                                                                        
  toString.call(a)      // else call Object.prototype.toString method
  .slice(8, -1)         // '[object Array]' -> 'Array'
  .toLowerCase()        // 'Array' -> 'array'
}

// type([]) === 'array'
// type({}) === 'object'
// type('') === 'string'
// type(1) === 'number'
// type(true) === 'boolean'
// type(function(){}) === 'function'
// type(null) === 'null'
// type(undefined) === 'undefined'

index.js

function(a){return a==null?a+'':toString.call(a).slice(8,-1).toLowerCase()}

LICENSE.txt

Copyright (c) 2011 Kir Belevich, http://soulshine.in
 
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
 
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
 
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

package.json

{
  "name": "type",
  "keywords": ["typeof", "type", "isArray"]
}

({}).toString.call(undefined)
"[object Undefined]"

({}).toString.call(null)
"[object Null]"

oops.
it's not so crossbrowser o_O, sorry.
your variant with a==null?a+'' works fine.

i think this is as good as it gets! maybe rename it to type to avoid confusion with the native operator?

you're right. renamed.

This will break in at least IE < 9 and isn't consistent for some objects in at least FF5. Using the window.toString should be ok on paper but isn't in practice. You should stick with {}.toString.call(...).