Why does CoffeeScript compile a for loop in this way?

Question

This piece of CoffeeScript:

for i in [1..10]
  console.log i

is compiled to:

for (i = _i = 1; _i <= 10; i = ++_i) {
  console.log(i);
}

I don't understand why it doesn't just use an i. Any ideas?

Answer 1

I'm not very familiar with CoffeeScript, but my guess is that it is to prevent modification of the i variable within the loop.

For instance:

for i in [1..10]
  console.log i
  i = 7

might have produced this code

for (i = 1; i <= 10; ++i) {
  console.log(i);
  i = 7;
}

This obviously produces an infinite loop.

CoffeeScript's version, however, means this happens:

for (i = _i = 1; _i <= 10; i = ++_i) {
  console.log(i);
  i = 7;
}

The loop is no longer infinite because of the presence of _i to track the position in the loop.