Hard Rock CoffeeScript

Loops and Comprehensions

Array iteration in JavaScript has a rather archaic syntax, reminiscent of an older language like C rather than a modern object-orientated one. The introduction of ES5 improved that situation somewhat, with the forEach() function, but that still requires a function call every iteration and is therefore much slower. Again, CoffeeScript comes to the rescue, with a beautiful syntax:

CoffeeScript

for surname in ["Young", "Gilmour", "Blackmore"]
  console.log "David #{surname}"

JavaScript

var surname, _i, _len, _ref;
_ref = ["Young", "Gilmour", "Blackmore"];
for (_i = 0, _len = _ref.length; _i < _len; _i++) {
  surname = _ref[_i];
  console.log("David " + surname);
}

If you need the current iteration index, you are able to pass an extra argument:

CoffeeScript

for count, i in ["one", "two", "three"]
  console.log "#{i} - #{count}"

JavaScript

var i, count, _len, _ref;
_ref = ["one", "two", "three"];
for (i = 0, _len = _ref.length; i < _len; i++) {
  count = _ref[i];
  alert("" + i + " - " + count);
}

You can also iterate on one line, using the suffix form by placing for at the end of expression

CoffeScript

like guitarist for guitarist in ["Young", "Gilmour", "Blackmore"]

JavaScript

var guitarist, _i, _len, _ref;
_ref = ["Young", "Gilmour", "Blackmore"];
for (_i = 0, _len = _ref.length; _i < _len; _i++) {
  guitarist = _ref[_i];
  like(guitarist);
}

And also filtering is available in comprehensions Python style:

guitarists = ["Young", "Gilmour", "Blackmore"]
like guitarist for guitarist in guitarists when guitarist[0] is "G"

You can also use comprehensions for iterating over properties in objects. Instead of the in keyword, use of.

CoffeeScript

names = Angus: "Young", David: "Gilmour", Richie: "Blackmore"
alert("#{first} #{last}") for first, last of names

JavaScript

var first, last, names;
names = {
  Angus: "Young",
  David: "Gilmour",
  Richie: "Blackmore"
};
for (first in names) {
  last = names[first];
  alert("" + first + " " + last);
}

The only low-level loop that CoffeeScript exposes is the while loop. This has similar behavior to the while loop in pure JavaScript, but has the added advantage that it returns an array of results, i.e. like the Array.prototype.map() function.