The difference between: if ((typeof OA! = 'Undefined') && OA) and if (OA)

Question

What is the difference between: if((typeof OA != 'undefined') && OA )and if(OA)?

The former expression works; the latter quietly stops the execution of the current function.

(maybe a rookie question)

Thank!

+3

user1279175 Apr 17 '12 at 17:07

3 answers

Rocket hazmat · Answer 1 · 2012-04-17T17:12:16+0000

if(OA)will fail if it OAhas never been determined. typeof OA != 'undefined'checks if exists OA.

var OA;
if(OA){
}

It works.

if(OA){
}

It does not work: OA is not defined.

typeof OA != 'undefined' && OA checks if this is defined before trying to access the variable

Amareswar · Answer 2 · 2012-04-17T17:12:33+0000

the compiler will not try to evaluate OA incase of typeof, where, as in it, it tries to evaluate if (OA)

jbabey · Answer 3 · 2012-04-17T17:20:34+0000

if ((typeof OA != 'undefined') && OA)

, OA. , .

if(OA)

This assumes that it exists OAand immediately translates it into a logical one and evaluates it.

The second example will throw a javascript exception if the variable OAhas never been declared - the first example avoids this.

See my answer here for a more detailed explanation of several values undefinedin javascript.