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Just starting out learning JS. I understand defining variables. Why leave one undeclared? Does it help when constructing if/then statements?

Kevin
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    This might be a duplicate: http://stackoverflow.com/questions/15985875/effect-of-declared-and-undeclared-variables – dbarnes Aug 26 '13 at 02:45

2 Answers2

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Why leave one undeclared?

function foo() {
    var i = 0; // local variable
    j = 1;     // global variable
}
foo();
i; // undefined
j; // 1
function bar() {
    var k; // local variable
    k = 2; // still local
}
bar();
k; // undefined

If foo is in "use strict" mode, it will cause a ReferenceError: j is not defined unless another j is defined higher up the scope chain, because there was no var for the j.

Paul S.
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  • as far as i know you cant declare a global variable **inside** a function and "j=1;" (if its not a error) will be a local variable – Math chiller Aug 26 '13 at 02:54
  • @tryingToGetProgrammingStraight If you don't believe it, try the code I've posted here, followed by a check to see if there is a global `j` and report back :-) I believe that the `j;` outside of the _function_ as in my example is sufficient proof, though. – Paul S. Aug 26 '13 at 02:56
  • It is true that `j` will not necessarily become global if there is a `var j` higher in scope but not globally, though. – Paul S. Aug 26 '13 at 02:59
  • your right i tried it – Math chiller Aug 26 '13 at 03:04
1

yep it can "help when constructing if/then statements?" the value of undefined is false. so:

 if ( myVar )

means if its got a value true if not false

but its best to do:

var myVar;

which still is false not declaring may give a error in a older browser

im not sure happens if you already have a global "var i;" and then try using one in a loop, i think it will just make you lose the global "i" for the new one.

Math chiller
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