On 09/12/2016 05:32 PM, Danielle McLean wrote:
In current ECMAScript, it is legal to place a variable declaration inside the
initialiser of a `for` loop, as well as to declare the variable used by a
`for...in` or `for...of` loop within the declaring expression:
for (let i = 0; i < 5; ++i) console.log(i);
for (let item of collection) process(item);
When this syntax is used with `let` or `const`, the resulting variable is
scoped to the loop and is not visible to the rest of the surrounding block.
I propose that this syntax be extended, making it legal to place a variable
declaration within the condition of an `if` or `while` statement. Any truthy
value will cause the `if` block to run or `while` loop to repeat, as usual -
the advantage is that the particular truthy value is bound to a variable and
can be used inside the conditional block.
My initial reaction was positive, but now I don't think it works.
First, other places in the grammar do not restrict let/const to a single
variable. Should
if (let a=0, b=1, c=0) { ... }
execute the if block or not? The obvious solution is to require a single
variable, which means the grammar for these let/consts is different from
others. What about
x = { a: 1 };
if (let {a} = x) { ... }
Second, that previous example makes it unclear to me at first glance
what the intended semantics *should* be. I could imagine this printing
either 1 or 2:
h = { foo: 0};
if (let {bar=1} = h) {
print(1);
} else {
print(2);
}
Is the conditional based on the variable's final value, or on whether or
not the destructuring found a match? I could argue for either one, so
even if there's a natural way to resolve my first problem, I think the
code looks ambiguous to the eye.
if (let { children } = node) {
print("interior node");
} else {
print("leaf node");
}
Again, the simplest way to resolve this is to restrict it to "let/const
IDENTIFIER = expression", but it feels weird to have different rules for
this particular case. for(let...) on the other hand, does not attempt to
use the let expression as a value, so it does not encounter any of these
problems.
As a minor issue, it also feels a little awkward to special-case this
conditional expression. I can do
if (let x = foo()) print(x)
but not
(let x = foo()) && print(x)
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