An "extend" operator is a natural companion to <|

Allen Wirfs-Brock allen at
Tue Jul 19 15:13:57 PDT 2011

On Jul 19, 2011, at 11:23 AM, Brendan Eich wrote:

> On Jul 19, 2011, at 11:07 AM, Bob Nystrom wrote:
>> // "new" lets you declare members on new instances. these would presumably be
>> // invoked on the new object before the constructor body is run.
>> new:
>>   numAttacks = 0;
>>   // declaring an instance property here mainly so you can document it. could be
>>   // useful later for guards or other annotations.
>>   name;
> The plain name; also looks like a useless expression-statement.

When we talked some people pretty strongly felt that it was very desirable to be able to explicitly declare the intended per instance properties.  In addition to document purposes such declaration might, for example, be used for things like linting for this.mispellings in the body of the function.  Such explicit  declarations are probably essential if they are private properties.  But I don't see any problem with requiring inclusion of an initial value, even it it is going to be |undefined.|

> Beyond this, I'm concerned that per-instance properties need initializers that use constructor parameters. So they ought to be initialized in the constructor.
> Whether they can be declared outside the constructor is a separate issue. Perhaps if they have constant or constructor-invariant initializers, they can be initialized where declared too.

This seemed to be a big issue in the past that caused a lot of thrashing about how to put such declarations inside the constructor.  Backing off from that requirement would be helpful.

> But declarations should look different from expressions. The alternative we keep returning to is the property assignment sub-grammar of object literals, which would want
>   new:
>     numAttacks: 0;
> with a semicolon not a comma.

some thoughts -- why does the punctuation following the new (or class or prototype or whatever).  For example, what about

     numAttacks: 0;

that looks more distinctly like a section break and different from a property name (and otherwise how would one define a property name new or class or prototype or private?)

Also, why ; instead of , like in object literals  (alternatively could we allow use of ; as a separator in object literals).  It seems desirable to have consistency of property definitions between object literals and class declarations.

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