ES6 plans

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New and improved builtin APIs

Most of this stuff does not need to touch a lot of code. None of it requires a Harmony opt-in. This is great stuff for new contributors to start work on.

• Simple Maps and Sets - The spec is incomplete but I will be landing what's there now in bug 697479.
• Binary data - bug 578700
• New reflection methods Object.getPropertyDescriptor, Object.getPropertyNames
• Number.isFinite, isNaN, isInteger, and toInteger
• String.prototype.repeat, startsWith, endsWith, contains, toArray
• New reflection API that complements proxies: harmony:reflect_api
• Object.is
• More math functions - Proposed are: Math.log10, log2, log1p, expm1, cosh, sinh, tanh, acosh, asinh, atanh, hypot, trunc, sign; perhaps gamma and erf; and perhaps randomInt(n). See bug 717379.
• ES6 will spec that Math.random does not share state across multiple globals.

New syntax (stuff that affects the front end and/or bytecode)

This stuff isn't terribly hard either, for the most part.

• for-of loops - bug 699565 - This is I'd say better than half done. It was easy. XPConnect must be updated so that arraylike XPCOM objects cooperate with for-of.
• Parameter default values
• Rest parameters - bug 574132
• Spread operator - bug 574130 - The operator can appear in two places: function calls and array literals. Maybe two bugs would make sense.
• Proper tail calls
• Extensions to object literal syntax
• Classes
• Quasi-literals Bug 688857
• Pragmas

Updating SpiderMonkey extensions to the ES6 spec

We've implemented a ton of stuff, dating back years, that is going to be in ES6 but in a slightly different form.

Direct proxies

There is a ton of verbiage here and it is still under active development. Still, I think it should be pretty easy to implement. We already have the forwarding code. We can keep Proxy.create and createFunction without a lot of extra effort.

Bug 703537.

• http://wiki.ecmascript.org/doku.php?id=harmony:direct_proxies
• http://wiki.ecmascript.org/doku.php?id=harmony:proxies_spec
• http://wiki.ecmascript.org/doku.php?id=harmony:proto_climbing_refactoring
• http://wiki.ecmascript.org/doku.php?id=harmony:virtual_object_api

Generators and comprehensions

• Certain scoping details in array comprehensions and generator expressions are probably different from what we have implemented.
• function* syntax - bug 666399
• Allow returning a value from a generator - bug 666404
• There are probably one or two other minor details - details of StopIteration, that kind of thing.

let, const, and block functions

Block scoped bindings

• Top-level let shouldn't be the same thing as var, per dherman: bug 589199
• TC39 is specifying that reading a let-variable before it is initialized is an error, which will be a separate bug.
• const - bug 611388
• Block functions - bug 585536 (note - opt-in only, still banned in ES5 strict mode)
• Destructuring
• Per loop iteration binding of let/const.

Harmony infrastructure

Some minimal steps here are a prerequisite to doing the features that require a Harmony opt-in: modules and import, block-scoped functions, typeof null, and lexical scoping of names declared at toplevel.

• New SM version number - This is enough to get started on harmony features with only shell tests.
• Opt-in via type="text/ecmascript;version=6" or whatever - bug 694107
• Per-script opt-in syntax ('use harmony;' or whatever)
• Opt-in via HTTP header?
• Whole-page opt-in via HTML feature?

Modules

harmony:modules

Modules are bug 568953.

• module syntax, scoping semantics, module objects
• import syntax
• from "url" syntax
• Hook up from "url" and async module loading to Gecko
• Module loaders

With a Harmony opt-in, programs get very different global binding behavior. The bindings are no longer shared with the global object's properties. This is a major change.

New APIs that are in modules

• Reorganize the standard library into modules
• Private name objects - bug 645416
• A few functions for iterating over object properties

Incompatible changes

Harmony may change some ES1-5 behavior, if it empirically turns out to be doable without breaking the Web.

• Change how eval decides which value to return
• typeof null == "null" with Harmony opt-in - bug 651251

Proposals that should not create any work for us

• Multiple globals
• RegExp y (sticky) flag
• RegExp match web reality
• Function.prototype.toString spec tweaks