Question

This teaspoon of Coffee …

    _pickInConf = (sourceConf,propsToPick...) ->
      newConfWithPickedProperties = {}
      newConfWithPickedProperties[key] = sourceConf[key] for key in Array::.concat.apply Array::,propsToPick when key in sourceConf
      newConfWithPickedProperties

… gets compiled into :

_pickInConf = function() {
  var key, newConfWithPickedProperties, propsToPick, sourceConf, _i, _len, _ref;
  sourceConf = arguments[0], propsToPick = 2 <= arguments.length ? __slice.call(arguments, 1) : [];
  newConfWithPickedProperties = {};
  _ref = Array.prototype.concat.apply(Array.prototype, propsToPick);
  for (_i = 0, _len = _ref.length; _i < _len; _i++) {
    key = _ref[_i];
    if (__indexOf.call(sourceConf, key) >= 0) {
      newConfWithPickedProperties[key] = sourceConf[key];
    }
  }
  return newConfWithPickedProperties;
};

… and makes use of :

__indexOf = [].indexOf || function(item) { for (var i = 0, l = this.length; i < l; i++) { if (i in this && this[i] === item) return i; } return -1; };

I expected Coffeescript compiler to transcompile this chunk :

when key in sourceConf

into :

if (key in sourceConf) {

… Which is what i meant to code in JS …

Is there a way i can force Coffescript to output this kind of JS ? Or to make it understand that sourceConf is an object, not an array ?

Thanks in advance for your answers !

Answer 1

You want of not in to iterate keys of an object: http://coffeescript.org/#loops (near the end of the section)

To expand on this:

for a in items when a of obj
  stuff

compiles to

var a, _i, _len;

for (_i = 0, _len = items.length; _i < _len; _i++) {
  a = items[_i];
  if (a in obj) {
    stuff;

  }
}

which is using the a in obj you are looking for