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 <rodrigol...@gmail.com> 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.
> _______________________________________________
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>
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