```
if (someComplicatedCondition()) {
doSomeLogic();
doSomeOtherLogic(if.value, true);
}
```
On Wed, Mar 21, 2018 at 2:52 AM, Rodrigo <[email protected]> wrote:
> Here are my gripes with `let` and `const` returning values:
>
> 1) declaration lists are hard to read:
>
> if ((let x = 10, y = 20) > 15) {
> // true, but what's being compared here? 10 or 20? (answer: 20)
> }
>
> Although right now this is allowed and the last element is compared:
>
> if ((x = 10, y = 20) > 15) {
> // result is true, 20 > 15
> }
>
> 2) Destructuring assignments are also confusing, what's being compared
> here?
>
> if(let [x,y] = [1,2]) {
> }
>
> Again, this is allowed as of today:
>
> if([x,y] = [1,2]) {
> // true, as it returns [1,2]
> }
>
> 3) Nesting `let/const` would be either expected everywhere (not only
> in the `if`) or a possible side effect from the implementation.
> Similar to languages such as Perl.
>
> let x = foo(let y = 100, z = 200); // what's the scope of x and z?
>
> This leads to hard to read and very confusing code golf.
>
> That's why Golang went with something simple,
> `if([declaration];[conditional])`, and avoided confusion over `:=`
> assignments returning values anywhere in the code. `x:=( y:= 20 )` is
> not allowed in Go.
>
> It expands on the 45-year tried-and-true structure of `for(;;)` to
> create `if(;)` and keep the ES language simple and clear expanding on
> its own concept of `for(;;)`.
>
> On Wed, Mar 21, 2018 at 7:57 AM, Naveen Chawla <[email protected]>
> wrote:
> > OK I neglected to read the original post fully. My last post example
> would
> > be based on allowing `const` and `let` declarations to expressions in of
> > themselves (in the case of multi variables, returning the last one). So
> let
> > me ask, what exactly would be the problem with this?
> >
> > On Wed, 21 Mar 2018 at 12:20 Naveen Chawla <[email protected]>
> wrote:
> >>
> >> What would `if.value` look like in an example?
> >>
> >> Wouldn't it be possible to have something like `if(const x = getX() &&
> >> const y = getY())` to capture more than just the conditional if
> required?
> >> I'm guessing that would probably break something somewhere, but I'm not
> sure
> >> what.
> >>
> >> On Wed, 21 Mar 2018 at 04:35 Jordan Harband <[email protected]> wrote:
> >>>
> >>> Is the use case only ever to capture the thing that serves as the
> >>> conditional?
> >>>
> >>> If so, would perhaps something like `if.value` work better? Since it's
> a
> >>> keyword, it could be made to only work in the `if` block, and you
> wouldn't
> >>> need any of that odd multi-statement stuff in the conditional parens.
> >>>
> >>> On Tue, Mar 20, 2018 at 12:57 PM, Rodrigo <[email protected]>
> wrote:
> >>>>
> >>>> Proposal: inline let/const statements to declare and initialize
> >>>> variables within if statements, so that temporary variables exist only
> >>>> within the if/else block scope.
> >>>>
> >>>> Reason: limits variable scope to the block where really needed, in
> >>>> similar fashion to variables defined in for(;;) statements. This
> >>>> improves readability while reducing unnecessary variables roaming
> >>>> outside their needed block.
> >>>>
> >>>> The syntax would be very similar to the for(;;) assignment/test pair:
> >>>>
> >>>> if (let x = 100; x > 50) {
> >>>> console.log(x); // 100
> >>>> }
> >>>> console.log(x); // ReferenceError
> >>>>
> >>>> // same for const
> >>>> if( const x = foo(); typeof x === 'object' ) {
> >>>> //...
> >>>> }
> >>>>
> >>>> // the variable is available within any else block
> >>>> // after its declaration
> >>>> if (let x = foo(); x < 50) {
> >>>> console.log(x); // y is not available here
> >>>> } else if (let y = bar(); y > 0) {
> >>>> console.log(x, y);
> >>>> } else {
> >>>> console.log(x, y);
> >>>> }
> >>>>
> >>>> Right now there isn't a way to limit a variable to the if block:
> >>>>
> >>>> let x = 100;
> >>>> if (x > 50) {
> >>>> console.log(x);
> >>>> }
> >>>> // x is in scope, but may not be needed beyond the if statement
> >>>> console.log(x);
> >>>>
> >>>> // or a non-strict assignment, which also "leaks" scope
> >>>> if( (x = 100) > 50 ) {
> >>>> // ...
> >>>> }
> >>>>
> >>>> There are many "workarounds" available, here's a few:
> >>>>
> >>>> // workaround 1: can be remedied with a scope block
> >>>> // but it's asymmetrical and non-idiomatic
> >>>> {
> >>>> let x = 100;
> >>>> if (x > 50) {
> >>>> console.log(x);
> >>>> }
> >>>> }
> >>>>
> >>>> // workaround 2: with a for statement
> >>>> // but this is non-idiomatic, hard to read and error-prone
> >>>> for (let x = 100; x > 50;) {
> >>>> console.log(x);
> >>>> break;
> >>>> }
> >>>>
> >>>> If-initialization is available in many languages (Go, Perl and Ruby
> >>>> come to mind) and are considered best practice in each one of them:
> >>>>
> >>>> // Golang - x is defined, assigned and conditionally tested
> >>>> if x := 100; x > 50 {
> >>>> // x is in scope here
> >>>> } else {
> >>>> // and in here
> >>>> }
> >>>> // x is not available here
> >>>>
> >>>> ###### Perl
> >>>> if( my $x = 100 ) {
> >>>> print $x;
> >>>> }
> >>>> print $x; # an error
> >>>>
> >>>> if ( ( my $x = myfoo() ) > 50 ) { # also ok in Perl
> >>>> print $x;
> >>>> }
> >>>>
> >>>> ###### Ruby
> >>>> if ( x = 100 ) # parens required per style guide
> >>>> puts(x)
> >>>> end
> >>>> puts(x) # unfortunately Ruby does not limit scope to if, so x
> >>>> "leaks"
> >>>>
> >>>> I think this would be a great and important addition to the language.
> >>>>
> >>>> -Rodrigo
> >>>>
> >>>> PS: Just for the sake of comparison, Perl-style if-assignments could
> >>>> also be an
> >>>> option, albeit a very bad one IMO:
> >>>>
> >>>> if( ( let x = 100 ) > 50 ) {
> >>>> }
> >>>>
> >>>> A Perl-style, value-returning let/const has readability issues, opens
> >>>> quite a few fronts and sort of implies that let/const can return
> >>>> values anywhere in the code outside if/else. On the other hand it
> >>>> would fit with the currently if assignment if( x = y ). Definitely not
> >>>> recommended.
> >>>> _______________________________________________
> >>>> es-discuss mailing list
> >>>> [email protected]
> >>>> https://mail.mozilla.org/listinfo/es-discuss
> >>>
> >>>
> >>> _______________________________________________
> >>> es-discuss mailing list
> >>> [email protected]
> >>> https://mail.mozilla.org/listinfo/es-discuss
> _______________________________________________
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>
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