Discussing about let & const support

[srenault]

Recently, you offer us let and const support. Sadly, the only way to use it is to target es6.
Why don't implement the logic in the compiler itself like others es6 features (class, arrow functions).
Does the reason is technical ?

[DanielRosenwasser]

There actually are semantic differences with let/const that prevent us from just performing compile-time verification and turning them into vars. For example, take the following two code samples:

for (let i = 0; i < 5; i++) {
    setTimeout(() => console.log(i), 200);
}

for (var i = 0; i < 5; i++) {
    setTimeout(() => console.log(i), 200);
}

The only difference is the use of let vs var. The latter is a notorious example of how closure capture can be counter-intuitive in ECMAScript, where you'll just get:

5
5
5
5
5

On the other hand, the let example will give you the "expected" behavior:

0
1
2
3
4

We could support it, but the codegen isn't always the prettiest, and causes a perf-impact. We'd like to first get ES6 parity before we prioritize down-level support.

[joewood]

Is this issue being tracked elsewhere? I'm wondering how Traceur handles this problem. ES6 support for let and const still seem inconsistent.

[basarat]

More technical discussion around let: #1690

Closure creation is how 6to5 and traceur handle it, and it does result in memory bloat even for them.