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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