null & undefined

The undefined value is returned in three cases:
* If you declared a variable but did not assign a value to it.
* If you access an undefined property of an object (and in JavaScript you can see anything you like as a property of an object).
* If you have defined a function argument, but no value is passed to it. (See 5I for arguments.)

0 || 100   // 0
1 || 100   // 1

document.getElementById('nosuchid') // return null
typeof (document.getElementById('nosuchid'))  // object, 'cause typeof null == 'obejct'
null||'test'                        // test

firebug中如果沒有印出任何值，那就是'undefined'
access an undefined property of an object

var foo = {'bar':'bar'}; 
foo.tttt // nothing happen, it's 'undefined'
foo.bar||'else'  // 'bar' 

window.ttttt        // nothing happen, it's 'undefined'
window.tttt||'else' // 'else'
typeof window.tttt  // 'undefined'
typeof window.tttt == 'undefined' //true
typeof window.tttt === 'undefined' //true

declare a variable but not assign value to it:

var tttt; 
tttt          // nothing happen, it's 'undefined'
alert(tttt)   // undefined
typeof tttt   // 'undefined'
alert(typeof tttt)  // undefined

not declare at all

tttt   // error: tttt is not defined
tttt == 'undefined' // error: tttt is not defined
tttt||'test'   // error: tttt is not defined
typeof tttt // 'undefined'
typeof tttt == 'undefined' //true
typeof tttt === 'undefined' //true