sample.coffee

console.log 'for in', "all: #{$scope.all}"
for user in $scope.participants
  console.log  user
  user.checked_out_events[$scope.id].selected = $scope.all

console.log 'forEach', "all: #{$scope.all}"
_.forEach $scope.participants, (user) ->
  console.log user
  user.checked_out_events[$scope.id].selected = $scope.all

sample.js

var user, _i, _len, _ref;

console.log('for in', "all: " + $scope.all);

_ref = $scope.participants;
for (_i = 0, _len = _ref.length; _i < _len; _i++) {
  user = _ref[_i];
  console.log(user);
  user.checked_out_events[$scope.id].selected = $scope.all;
}

console.log('forEach', "all: " + $scope.all);

_.forEach($scope.participants, function(user) {
  console.log(user);
  return user.checked_out_events[$scope.id].selected = $scope.all;
});