typeof usage for undefined variables

Question

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

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

Thanks

Answer 1

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 (ie use === not == ).

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

Answer 2

使用严格比较（ === ），并引用"undefined" ：

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