[css-font-loading] [info discovery] iterable doesn't tell you what index you are at

[litherum]

One natural use case use case of the information discovery facilities in css-font-loading would be "I want to swap the green color in my palette to my custom color." Doing this requires 2 steps:

1. Iterating over the colors in the palette until you find green
2. Creating a @font-palette-values rule that maps the index of green to your new custom color

The trick here is the "index" of green. When you iterate through an iterable, it doesn't tell you what index you're at. So, if you want to determine what index green lies at, you have to track that yourself.

You can do this with a 3-clause for loop, and getter, but maybe we can do better here.

[tabatkins]

While it's not the default iterator, you can iterate over the .entries() iterator to get the index and the value at the same time. All three of the iterator methods are provided by iterable<>.

[litherum]

Does entries() actually produce the index and the value at the same time?

[tabatkins]

Yeah, .keys() produces an iterator for the keys (numeric indexes in this case), .values() (the default for arrays) produces just the values, and .entries() produces [index, value] pairs.

You can then conveniently destructure over them:

for(let [i, v] of palette.entries()) { ... }

[litherum]

https://heycam.github.io/webidl/#idl-iterable

If a single type parameter is given, then the interface has a value iterator and provides values of the specified type. If two type parameters are given, then the interface has a pair iterator and provides value pairs with the given types.

Currently, the spec just has iterable<DOMString>, not iterable<unsigned long, DOMString>. Maybe I'm misunderstanding?

[tabatkins]

That's describing the default iterators, yes. Look slightly further down:

Interfaces with iterable declarations must not have any interface members named "entries", "forEach", "keys", or "values", or have any inherited interfaces that have members with these names.

This is because those four methods are automatically provided by the iterable declaration (with the specifics defined in https://heycam.github.io/webidl/#es-iterable)