typeof usage for undefined variables

what is the best usage for the "typeof" JavaScript function?

if (ty开发者_如何学Gopeof (myvar) == 'undefined') { 
//or
if (typeof (myvar) == undefined) { 
//or
if (typeof myvar == 'undefined') { 
//or
if (typeof myvar == undefined) { 

Thanks

typeof is an operator, not a function, and returns a string; so do not use parentheses and do compare it to a string.

When you compare things, avoid type coercion unless you need it (i.e. use === not ==).

if (typeof myvar === 'undefined') { 

Use strict comparison (===), and quote "undefined":

if (typeof myvar === "undefined") {}