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Comment by couchand | Hacker News Reader

I've been spending some time thinking about CoffeeScript's choice for `in` and `of` and I think I have the answer: they should be more regular. One (probably `of`) should be used for the keys/indexes and the other (probably `in`) should be used for contents.

These are legal CoffeeScript examples:

    for key, value of object
    if key of object

    for value in array
    if value in array

    for index, value of array
    if index of array

The one that's missing is:

    for value in object
    if value in object

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The issue with coffeescript is that it uses the opposite syntax to JS - `in` for arrays instead of objects. ES6 introduces the `of` keyword for real iterators so i don't really want to use that for either of them. It's tricky. There are other options but they're verbose

    every element as key, value from arr {
      console.log(key, value);
    }

    every property as key, value from obj {
      console.log(key, value);
    }

Thoughts on this?

    every [key, value] from arr {
      console.log(key, value);
    }

    every {key, value} from obj {
      console.log(key, value);
    }

    every [key] from arr {
      console.log(key);
    }

    every {_, value} from obj {
      console.log(value);
    }

Looks a bit cramped to me but I get the impression that array comprehension is one of the major reasons that people use Coffeescript, so perhaps it's not too much trouble?

There's also the issue with some fonts making it hard to differentiate between { and [ but it's an idea that might be worth to think about at least.

I quite like this suggestion. If we flip the key, value around to make it inline with what forEach etc do, then we don't need the placeholder for iterating without the key:

    fast-each [value] from arr {

    }

    fast-each {value} from obj {

    }
 
    fast-each [value, key] from arr {

    }

    fast-each {value, key} from obj {

    }

The disadvantage is that this looks like array comprehension but isn't.

Alternative:

    fast-properties key, value from obj {

    }

    fast-elements index, value from arr {

    }

    fast-properties value from obj {

    }

    fast-elements value from arr {

    }

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