Tab completion for block-scoped variables doesn't work in REPL

[RB14]

Using version 1.4.1 REPL mode, it seems that tab completion for block-scoped variables (i.e. let and const statements) doesn't show suggestions, while tab completion for variables declared with var or otherwise just assigned a value without a deceleration, does show up a list of suggestions.

Of course, this happens in strict mode, or else I couldn't use let and const.

[vkurchatkin]

A test (run with --use_strict):

require('assert').equal(require('vm').runInNewContext(`
  let a = 42;
  this.a
`), 42);

summoning @domenic.

[benjamingr]

I can reproduce this on v1.4.1 on OSX.

$ ./iojs --use_strict

let foobar = 40;
var bazbar = 100;
foo // hit tab, no autocomplete
baz // hit tab, autocompletes to bazbar

The test @vkurchatkin posted also fails.

[bnoordhuis]

I think this can be explained because of how the REPL does autocompletion: it spawns a nested VM context that contains a copy of the globals from the parent context. var bindings have global scope but let bindings don't, i.e.:

> var x = 1; typeof global['x']  // 'number'
> let y = 1; typeof global['y']  // 'undefined'

I can't think of a good fix off the top of my head.

[vkurchatkin]

the question is, this the correct behaviour according to spec?

[benjamingr]

@vkurchatkin it appears that it is.

[domenic]

Right, indeed, let and const and class don't add to the global object, but instead to a separate scope contour. Unsure how to deal with this given the implementation details @bnoordhuis mentions.

[Fishrock123]

Is this actually an issue then?

[RB14]

Well, as the one who opened this ticket, I still find it odd that auto-complete in REPL would work for only var (or should I say old-style) variables, and not for the new kinds of declarations.
Since auto-complete is a feature of io.js (and node.js), and it's a very useful feature if you ask me, I think this behavior should be fixed (if possible of course).. so I think it should still be considered as an issue.