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Comment by titzer | Hacker News Reader

1.) is simply wrong. Const variables have a live-range hole where access to the variable either throws or returns undefined, depending on the language mode (and the VM!). In either case it not legal to just smash in the constant.

2.) That's nice but JS semantics do not actually hold up. The intent is for a constant, the reality is there is a small window where the wrong results can be observed. Engines don't actually do any more optimizations than for var or let; in fact, your code may actually be slower due to the live-range hole checks.

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So you are saying that there are race conditions currently? Are any of this race conditions actually required by the language spec? If not, I suspect that those race conditions will be eliminated.

I believe that what the parent meant was that this:

function f() { console.log(a); const a = 1; console.log(a); }

...actually returns this:

undefined 1

So you get partially compile-time semantics (you can't assign to a) but partially run-time semantics (a only gets its value when initialised). Simply propagating the value of a's initialiser to all places where the variable is used is wrong, as this behaviour's required by the spec.

Actually that code example should throw an exception when the first `a` is evaluated.

It doesn't in v8 -- what do you mean by `should`?

It's an implementation error. In Firefox, it correctly throws:

> ReferenceError: can't access lexical declaration `a' before initialization

No, legacy const is not spec'd by ES5. It behaves differently in FireFox and V8 because it was added at different times with different semantics in the live range hole case. In strict mode, it will throw on V8 as well.

No it shouldn't. `a` at the first console.log invocation IS undefined, so it is entirely correct.

Not according to the spec[1]:

> The variables are created when their containing Lexical Environment is instantiated but may not be accessed in any way until the variable’s LexicalBinding is evaluated.

I.e. given a variable declared with `const`, you may not access it before it is set.

[1] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-let...

Consts have other purposes: Speeding up global constants using fixed field optimization https://blogs.windows.com/msedgedev/2015/05/20/delivering-fa...

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