Reference impl: Use arrow functions for size functions

[ninevra]

The reference implementation uses function expressions to create the size() functions for CountQueuingStrategy and ByteLengthQueuingStrategy, which results in them being constructible and having prototype properties. This PR changes to using arrow functions to better conform to the spec; see original discussion below:

Printing

new ByteLengthQueuingStrategy({ highWaterMark: 100 }).size.prototype

gives undefined in Chrome and Firefox. The Streams spec calls CreateBuiltinFunction without calling MakeConstructor, which is the abstract op that adds the prototype property. So it would appear that we should in fact use arrow functions after all

Originally posted by @TimothyGu in jsdom/jsdom#3200 (comment)

[ricea]

This doesn't set the name property. I'm not sure if it's sufficient just to assign it after creating the function. @domenic what's the best thing to do here?

[MattiasBuelens]

Chrome and Firefox do seem to infer a name property from the variable assignment. But I don't think ECMAScript requires that, so it's probably not a good idea to rely on it for the reference implementation.

const size = () => 1;
size.name // -> "size"

[TimothyGu]

ECMAScript does indeed require the name property to be set. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/name#inferred_function_names.

[ninevra]

I do find this to be very odd and unexpected behavior, but it is required by ecmascript.

Relevant part of the spec, from 14.3.1.2 Runtime Semantics: Evaluation of let or const bindings:

3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    a. Let value be NamedEvaluation of Initializer with argument bindingId.

capturing bindingId and passing it into the evaluation of the anonymous function expression, where it's eventually passed through to 15.3.4 Runtime Semantics: InstantiateArrowFunctionExpression, and then to SetFunctionName.

[ricea]

Thanks!

[ricea]

@ninevra Sorry for the trouble, but could you sign the CLA? https://participate.whatwg.org/agreement-status?user=ninevra&repo=streams&pull=1142

[ricea]

Optionally, if you could add a test to https://github.com/web-platform-tests/wpt/blob/master/streams/queuing-strategies.any.js that fails without this change it would be a big help.

[ninevra]

@ricea I think I've already signed the CLA; it says I need to be verified?

I'll try to get a test submitted today.