Question

The following CoffeeScript code:

for a in arr
  do_something_with arr.length

is compiled to:

var a, _i, _len;

for (_i = 0, _len = arr.length; _i < _len; _i++) {
  a = arr[_i];
  do_something_with(arr.length); // I want to use '_len' here
}

Is that possible to use the cached value of arr.length rather than calculating it in every iteration?

Answer 1

If you have a defined array and not a ranged array, then you could actually use _len in the block:

for a in arr
  _len == arr.length

however, this relies on the CS internals, which could change in a new version.
It also does not work for every array.

arr = [0..10]
for a in arr
  #this works
  console.log _len

but:

for a in [0..10]
  #this doesn't work
  console.log _len

I think if you want to used a cached version of the array length, it is better to do that explicitly in your code and not depend on the internal variable names of CS.

rather do:

arrLength = arr.length
for a in arr
   doSomethingWith arrLength