javascript variable scope in function confusion -

here 2 javascript functions

var = 10; function abcd() {  alert(a);//alerts 10  a=5; } 

and code this

var = 10; function abcd() {  alert(a);//alerts undefined  var a=5; } 

in both function assignment/declaration after alert() call. why alert message 10 , undefined respectively?

that's because variable gets "hoisted" of containing scope interpreter when declare it. code ends being interpreted this:

function abcd() {  var a;  alert(a); //alerts undefined  = 5; } 

to avoid kind of confusion, can follow practices keep things in place, declaring local-scoped (that is, variables declared keyword var inside function scope) variables right in beginning of function.

note that, can read article, happens nested functions aswell.