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.
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>>>
>>>
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