Support for Node 4

[Rich-Harris]

Due to laziness on my part, Svelte currently only runs in Node 6 and above. We should transpile it to ES5 using either Babel or Bublé (Bublé would be faster and require less config, but we would have to rewrite code that uses for ( const x of y ) {...})

[jacobmischka]

I've seen you use dangerousForOf in projects before, what's different about the loops in svelte that doesn't allow that?

[Rich-Harris]

Technically, for ( const x of y ) {...} assumes y is an Iterator. To transpile it correctly, you have to add a lot of code with nested try-catch statements which is less than ideal, and you have to polyfill the Symbol object.

Bublé is focused on generating lean code that runs fast, so it doesn't let you do that. The dangerousForOf transform is a half-way house that allows you to use for-of loops as syntactic sugar over array iteration:

for ( var i = 0, list = y; i < list.length; i += 1 ) {
  var x = list[i];
  // ...
}

In Svelte we're using for-of with a tokenizer that implements the Iterator protocol, rather than an array-like object. So dangerousForOf doesn't work. It'd be easy enough to rewrite those sections to not use for-of though.

[jacobmischka]

Oh I see, interesting. When using buble I'd always just used it straightaway and it seemed to work, so I wasn't sure exactly its conditions. Maybe I should go back and check now though.

[leebyron]

Hey Rich - just stumbled on this issue - I had a fork of buble sitting around that I was using for node 4 support for some private project that fixed this issue - I just wrote up a PR for it at https://gitlab.com/Rich-Harris/buble/merge_requests/109

[PaulBGD]

So what do you want to do for this? Just remove all cases of const x of y ?

[Rich-Harris]

I think so, IIRC — there might be some other stuff that doesn't work in Node 4 (or older browsers), but it might be just that