为什么循环箭头不能将胖箭头绑定到当前环境？

Question

If I compile this CoffeeScript:

    funcs = ((=> console.log i) for i in [0..2])                                                                                                                                                                                          

    funcs[0]()  // Prints 3
    funcs[1]()  // Prints 3
    funcs[2]()  // Prints 3

it produces this JavaScript:

    (function() {
      var funcs, i;

      funcs = (function() {
        var _i, _results,
          _this = this;
        _results = [];
        for (i = _i = 0; _i <= 3; i = ++_i) {
          _results.push(function() {
            return console.log(i);
          });
        }
        return _results;
      }).call(this);

      funcs[0]();

      funcs[1]();

      funcs[2]();

      funcs[3]();

    }).call(this);

I would think it would instead have:

          _results.push((function(i) {
             return function() {
              return console.log(i);
          }})(i));

Could someone explain why it's not doing that?

Answer 1

Fat arrow binds this lexically, not every variable. Use do to capture variables using an IIFE.

funcs =
  for i in [0..2]
    do (i) ->
      -> console.log i

Answer 2

This is not how closures work. You get a closure when you return a function from another function scope:

var closure = (function(i) {
    return function() {
        console.log(i);
    };
})(i);

This can be achieved via this ugly CoffeeScript (no need for a fat arrow here):

funcs = ((do (j=i) -> -> console.log j) for i in [0..2])

I'd recommend to not write this as a single line though.

Answer 3

Okay, so after looking at the CoffeeScript documentation, I believe I've answered my own question. => only binds the function to the current value of this . I had mistakenly assumed it bound all variables to their current value.

...although it would be neat if it had a feature like that. Maybe ->> ?

Answer 4

When you convert that coffee to javascript, you're getting a function that IS binding the scope. The problem is that:

funcs = (function() {
    var _i, _results,
      _this = this;
    _results = [];
    for (i = _i = 0; _i <= 3; i = ++_i) {
      _results.push(function() {
        return console.log(i);
      });
    }
    return _results;
  }).call(this);

in there, the variable i is bound, but by the time you call the function the value is 3, and remains in 3 every time you call the function.