mathiasbynens / especially

Abstract operations and other functions drawn from the ECMAScript specification

ECMAScript Spec Operations, in ECMAScript

Have you ever wanted to write a to-the-letter implementation of some ECMAScript spec function? Maybe you want to run tests against it; maybe you're just crazy and like self-hosting JavaScript in JavaScript. In either case, this is the package for you!

Especially has a small-but-growing collection of meta-textual and abstract operations drawn directly from the pages of the ECMAScript 6 draft specification. From common things like Get to dealing with internal slots, Especially has you covered.

APIs

Especially has no main module (gasp). Instead, you'll require one of the top-level modules that contain the stuff you want.

require("especially/abstract-operations")

• ArrayCreate
• CreateDataProperty
• CreateFromConstructor
• EnqueueTask
• Get
• GetIterator: uses the @@iterator symbol.
• GetMethod
• GetPrototypeFromConstructor
• HasProperty
• Invoke
• IsCallable
• IsConstructor: not spec compliant, but the best we can do
• IsPropertyKey
• IteratorComplete
• IteratorNext
• IteratorStep
• IteratorValue
• ObjectCreate
• OrdinaryConstruct: use this to get ES6 new semantics, with @@create support.
• OrdinaryCreateFromConstructor
• SameValue
• SameValueZero
• ToBoolean
• ToInteger
• ToLength
• ToNumber
• ToObject
• ToString
• Type: returns the types as strings, e.g. "Object".

require("especially/math")

A variety of math operations from the Algorithm Conventions section of the spec:

• abs(x)
• sign(x)
• min(x₁, x₂, …, x_n)
• floor(x)

require("especially/meta")

• assert: ensures you pass it a boolean, then throws if it's not true
• define_built_in_data_property: a shortcut for defining a built-in data property with the usual property descriptor.
• Internal slot management:
  • make_slots(object, arrayOfSlotNames): call this to initialize an object's internal slots. Often referenced in the spec as "instances of (something) are initially created with the internal slots listed in (some table)."
  • get_slot(object, name): get the value of an internal slot. Often referenced in the spec as "the value of (object)'s [[SlotName]] internal slot."
  • set_slot(object, name): set the value of an internal slot. Often referenced in the spec as "Set (object)'s [[SlotName]] internal slot to (a value)."
  • has_slot(object): check whether an object has an internal slot with the given name. Often referenced in the spec as "If (object) does not have a [[SlotName]] internal slot."

require("especially/well-known-symbols")

A few well-known symbol are included:

• "@@create" will give you a symbol that is used by OrdinaryConstruct and related operations.
• "@@iterator" will give you a symbol that is used by GetIterator.

require("especially/intrinsics")

Some of the well-known intrinsic objects are included by name, e.g. "%ObjectPrototype%". These are used by GetPrototypeFromConstructor and related operations.

Usage Notes

Especially is meant to run in Node.js 0.11.9+, at least for now. It uses certain ES6 features so far only implemented in bleeding-edge V8, with the --harmony flag turned on.

Install it from npm into your project with npm install especially. And don't forget to run your program, or your tests, with the --harmony flag.

You can see examples of it in use in the reference implementation of the ES6 promises specification.