less minimalism, richer leather

minimalist-classes.js

// A response to jashkenas's fine proposal for minimalist JavaScript classes.

// Harmony always stipulated classes as sugar, so indeed we are keeping current
// JavaScript prototype semantics, and classes would only add a syntactic form
// that can desugar to ES5. This is mostly the same assumption that Jeremy
// chose, but I've stipulated ES5.

// Where I part company is on reusing the object literal. It is not the syntax
// most classy programmers expect, coming from other languages. It has annoying
// and alien overhead, namely colons and commas. For JS community members who
// don't want classes, either proposal is "bad", so let's focus on the class
// fans who will use this feature.
//
// Therefore I propose giving classes their own new kind of body syntax, which
// is neither an object literal nor a function body. The main advantage is that
// syntax for class elements can be tuned to usability, even as it desugars to
// kernel semantics drawn from ES5 extended by both the super and private names
// proposals.

// Not shown below is the super proposal at
//
// http://wiki.ecmascript.org/doku.php?id=harmony:object_initialiser_super
//
// which composes as expected when 'super' is used in methods defined in a
// class instead of in an object literal.
//
// In contrast, you will see hints of the private name objects proposal at
//
// http://wiki.ecmascript.org/doku.php?id=harmony:private_name_objects
//
// Also visible throughout: the method definition shorthand syntax proposed at
//
// http://wiki.ecmascript.org/doku.php?id=harmony:object_literals#object_literal_property_shorthands
//
// For programmers who want classes as sugar for constructors and prototype, I
// hope this proposal wins.
//
// To be perfectly honest, I personally would be happy with Allen Wirfs-Brock's
// "Exemplars" approach and no classes. But for programmers coming from classy
// languages, I believe "minimalist classes" will tend to be too minimal to be
// usable out of the box. They'll want more.
//
// Brendan Eich, 1-Nov-2011

// First, basic usage from a real-world library (Three.js)

class Color {
  constructor(hex) {
    ...
  }

  // Prototype data properties, introduced by var or const at class body level.
  // Rationale: object literal syntax requires comma separation and has no way
  // to define a const that's clearly in Harmony. Classy languages do not need
  // no steenkin' commas, but we do want const in ES.next.
  //
  // An objection even I have made: we should not conflate property definition
  // syntax with lexical binding declaration syntax. But the cat's out of the
  // bag with 'var' in global code in ES1-5, and it's hard to beat "const" as
  // the keyword for initialize-only data properties.
  //
  // One fair objection: this syntax may mislead people into thinking methods
  // lexically close over prototype properties declared this way. A calculated
  // risk, what can I say?

  // Since, as far as I can see, class bodies are only declarative object
  // shape definitions, I think we can safely get rid of `var' for declaring
  // public slots in the class, just as we do with methods (or are they plain
  // functions?)
  r = 1;
  g = 1;
  b = 1;

  // That could be easily coupled with the comma-operator, I guess, but I'm
  // not that familiar with the grammar, so feel free to correct me.
  r = 1, g = 1, b = 1;

  // Or non-initialised slots, just like you have with `private':
  r, g, b;


  copy(color) {
    ...
  }

  setRGB(r, g, b) {
    ...
  }

  setHSV(h, s, v) {
    ...
  }

}


// To create a class with its prototype chain set correctly:

class Fox extends Animal {
  ...

  // The special syntax for defining class-level properties and methods uses a
  // prefix keyword. The 'static' keyword is reserved and somewhat hated (or at
  // least considered a misnomer), but I went with it instead of 'class' to
  // avoid confusion in overloading the c-word, and to be more future-friendly
  // in case nested classes are wanted (right now I do not want them, since a
  // class at class-body level would declare a prototype inner class and not a
  // static inner class).

  static const CONSTANT = 42;

  static classMethod() {
    ...
  }
}

console.log(Fox.CONSTANT);

Fox.classMethod();


// Classes are expression forms as well as declaration forms. As with function
// expressions, the name is optional in a class expression.

const frozenClass = Object.freeze(class {...});

// Named class expressions are supported too. The name is bound only in the
// scope of the class body, as for named function expressions.

animals.push(class Fox {});

// An anonymous class expression with superclass specification, after Jeremy's
// gist, but with an explicit and required body.

var subclass = function(parent) {
  return class extends parent {
    ...
  };
};


// Unlike Jeremy's proposal, classes cannot be built up programmatically by
// abutting an expression to a class head. That's too dynamic, it doesn't work
// with the 'super' proposal.  So any synthesis of a class body must use eval
// or equivalent. At this time, I do not propose adding a Class constructor
// analogous to Function, but I believe it could be added without issue.


// As in Jeremy's gist, here's the Monster class from the harmony: proposal
// (http://wiki.ecmascript.org/doku.php?id=harmony:classes#the_proposal_in_a_nutshell)
// ... sans unnecessary 'public' keyword prefixing!

class Monster {

  constructor(name, health) {
    this.name = name;
    this.health = health;
  }

  attack(target) {
    log("The monster attacks " + target);
  }

  isAlive() {
    return this.health > 0;
  }

  setHealth(value) {
    if (value < 0) {
      throw new Error("Health must be non-negative.");
    }
    this.health = value;
  }

  numAttacks = 0;

  const attackMessage = "The monster hits you!";

}

// Now there's one more problem to address, which could be deferred, but which
// falls under "batteries included" to some (not all) of the cohorts of classy
// programmers using JS or coming to it fresh in the future. And that is the
// cost of this[kPrivateName] given const kPrivateName = Name.create(...) where
// module Name from "@name" has been declared.
//
// Instead I propose we support private kPrivateName, ...; as a special form
// only in class bodies (for now) that both creates a private name object and
// binds it to a lexical const binding that may be accessed on the right of @
// in methods defined in a class.
//
// For class-private instance variables, obj@foo is supported as well (with no
// LineTerminator to the left of @ in this case, or it's short for this[foo]).
// See the new sameName method for an example of infix and prefix @ in action.
//
// There is no const instance variable declaration. Non-writable instance vars
// (properties to most people) are quite rare. Use ES5's Object.defineProperty
// if you must. This avoids ugly fights about read before constant instance var
// initialization too.

class Monster {

  private name, health;

  constructor(name, health) {
    @name = name;
    @health = health;
  }

  sameName(other) {
    return @name === other@name;
  }

  attack(target) {
    log("The monster attacks " + target);
  }

  isAlive() {
    return @health > 0;
  }

  setHealth(value) {
    if (value < 0) {
      throw new Error("Health must be non-negative.");
    }
    @health = value;
  }

  numAttacks = 0;

  const attackMessage = "The monster hits you!";

}

// As in the harmony:classes proposal, 'const class' freezes all of
//  1. the class binding, if named declaration rather than expression,
//  2. the class prototype induced by the const class,
//  3. the constructor method of the const class.
//
// 'const class' also seals instances constructed by the constructor, as if the
// last line of the constructor method were Object.seal(this), called using the
// original values of Object and Object.seal.

const class Frosty {
}

The main problem with skipping var and initializer (which we allow in Traceur) is that empty, non initialized properties look out of place:

class Point {
  x;
  y;
  ...
}

and it gets even worse without the optional semicolons:

class Point {
  x
  ...
}

I also think var fits nicely with const.