Suggestion: Support ES6 'let' and 'const' keyword

[RyanCavanaugh]

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-let-and-const-declarations

The only major implication here how we would treat export const x = 4; in the context of an internal module, as this basically introduces the concept of a readonly variable. See #12

[RyanCavanaugh]

What are the type system implications?

[electricessence]

I would state that there's two sides to this:

1. The target language is JavaScript and without the use of ES5 accessors or something greater than ES5, you're not really getting any benefit here in the end code.
2. const and readonly in C# do not behave the same from a compile stand point. const should exist statically or in an instance but must be initialized before the constructor can only be set by the definition. Where as readonly must be initialized before the constructor finishes.

Personally I like the idea of having const and readonly as an option. It may make things more complicated, but I like that it is explicitly declaring intent. Also, currently, it isn't possible to ensure an initialized value before the constructor is called.

[basarat]

👍

[wagle]

I've inherited a very large firefox add-on, written largely in javascript, that uses const and let all over the place. I'm trying to use typescript to debug, annotate, and refactor this legacy code. It seems a shame to remove "const", and I'm currently unsure how to remove "let". What's the status of including these features? I'm confused by the move from codeplex to github, the threads seem to dissipate.

How can I help? I really need this add-on to work right.

[danquirk]

Can you share what add on it is? We've been prioritizing ES6 features for which we can generate ES5 compatible code under the assumption that there's more value there until ES6 support is more prevalent in browsers (and thus more prevalent in JavaScript apps).

[wagle]

My copy is at https://github.com/wagle/tagspace

[wagle]

Apparently const and let have been in mozilla's internal javascript forever.

[duncanmak]

I think @wagle brings up another good point.

Like I wrote in #595, IMHO it's a quicker path and possibly more immediately useful if there's an additional code-gen path to emit non-ES5 (i.e. ES6, or in @wagle's case, perhaps JS2?) code.

[danquirk]

Newer browsers do tend to support let/const (http://kangax.github.io/compat-table/es6/) but as you can see it's by no means ubiquitous yet (ex iOS8 has no support for let, nor does IE10). We don't expect JavaScript developers en masse to take a dependency on those features until they're more common. Given that, we've been focusing our near term efforts on the ES6 features which we can give people the ability to leverage whether or not the browser supports them yet (ie can emit ES5 compatible code that models the ES6 semantics). That said, ES6 specific features like let/const are certainly important to us.

[wagle]

Yeah, I guess I need something like "--target js2", and no-one seems anxious to implement the mozilla javascript target.

In theory, I'm capable of adding const and let to your transpiler (5 semesters of compilers, years ago), but I'm not yet a "Javascript language lawyer", so I'm uncomfortable. I'm desperate to get this plugin debugged and extended this fall, so it looks like I'm going to try adding these keywords.

Adding const seems moderately straightforward, just look for assignments in the scope.

I'm not entirely sure what exactly let does yet (give me a day or two), but it just seems to limit the scope of what might be a var even more (correct me if I'm wrong).

(1) Would you need me to translate these to ES5 (or all the targets), or can I just disallow it?

(2) I might find that my plugin uses more than const and let, so I'd need those to. Would you expect me to implement all of "js2"? Or do I fork because I didn't do it all?

(3) Oh. Is the intention for there to be only one source language (typescript), but multiple targets (dialects of javascript)?

[mhegazy]

The way we would approach this is adding a new ES6 target. let and const will only work in ES6 but disallowed in other targets. An example to follow would be get and set accessors under ES3.

TypeScript is intended to be a super set of JavaScript, so the first thing to do here would be to go through the ES6 spec for let and const (http://wiki.ecmascript.org/doku.php?id=harmony:specification_drafts).

Let and const provide block scoping, const adds an additional constraint of being readonly.

If there are other ES6-specific features that you need let's take them on a case-per-case basis.

let me know if you have other questions.