On 1/15/11, And Clover <and-...@doxdesk.com> wrote:
> On Fri, 2011-01-14 at 10:12 -0800, cancel bubble wrote:
>> I understand that it's good practice to declare all your vars at the
> top of
> your function
>
> I disagree. This is one part of Crockford Dogma I think is badly
> mistaken. It's a habit from other languages that is, IMO, actively
> unhelpful in JS.
>
> function doSomething(things) {
> // Loop that does something
> for (var i= 0; i<things.length; i++) {
> var thing= things[i];
> thing.doPartOfSomething();
> }
>
> // Loop that does something else
> for (var i= 0; i<things.length; i++) {
> var thing= things[i];
> doAnotherPartOfSomething(thing);
> }
> }
>
> Here's two independent usages of the same variables `i` and `thing` in
> the same function. A purist might argue that you shouldn't reuse a
> variable in the same function, but in reality the alternatives are
> usually worse:
>
> - choosing different names each time means names have to be longer
> and/or less natural;
>
> - putting each independent or semi-independent block in its own function
> leads to an explosion of tiny unmanageable functions.
>
Well that's just what I'd do. Each loop a well named function. Or make
a "thing iterator" function.
> So with the recommendation to move the `var` to the top:
>
> function doSomething(things) {
> var i, thing;
>
> // Loop that does something
> for (i= 0; i<things.length; i++) {
> thing= things[i];
> thing.doPartOfSomething();
> }
>
> // Loop that does something else
> for (i= 0; i<things.length; i++) {
> thing= things[i];
> doAnotherPartOfSomething(thing);
> }
> }
>
> But now there's a gap between declaration and usage, one that gets more
> impractical the longer the function gets. And there's unnecessary
> coupling between those previously-independent blocks.
Those aren't problems when a separate function is declared.
If during a
> refactoring, you cut-and-paste one of the loops out of `doSomething`
> into another function, you've got yourself an accidental global with all
> the problems that incurs.
>
Well that would be an annoying mistake.
> So, I'd say: don't move all `var` declarations to the top after all.
> Instead, declare `var` every time you first assign a variable, not
> don't care if it existed previously.
When I feel like I need to add an additional section for variables in
a function, I start thinking about making a second function. So when I
have
var a, b, c;
p(a);
if(b) {
} else {
var d, e, f;
}
- then I consider to put the body of the `else` into a separate
function. Doing that can result in more functions that are shorter and
simpler and have more specific names. I always like it when I can read
the name of the function and guess exactly what it is going to do, and
then look at the function body and quickly see that I guessed right.
And if it matters, it is possible to make Crockford not be happy by
declaring the variables of the short functions at the top of those
functions. (notice that I did not say that it was possible to make
Crockford happy).
Garrett
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