typeof null -> "object"
typeof undefined -> "undefined",
-- so a=null, b=undefined; -> a==b, but a!==b, if(!a) true, if(!b) true;

typeof {} -> "object"
typeof [] -> "object"
-- so if({}), if([]) all true; if({}.length), if([].length) all false

typeof "0" -> "String"
typeof 0  -> "number"
typeof ''  -> "string", Empty String: boolean false
-- so if(0), if('') all false

typeof false -> "boolean"
typeof true -> "boolean"

//To check an object for null by look at it's length:
if(myObject.length==0){//means it is null.}
//in jQuery is:
if ( $('#myobject').length ){}

1.
var c;
-- if(!c) -> true
--> if(d) or if(!d)  -> error: ReferenceError: d is not defined

2. the following is different!
var a=undefined;
-- if(a==undefined) or if(a===undefined) -> true
-- if(typeof a==='undefined') or if(typeof a == 'undefined')
     -> true

-- if(d == undefined ) or if(d === undefined )
     -> error: ReferenceError: d is not defined
-- if(typeof d == 'undefined' ) or if(typeof d === 'undefined' )
     -> true