if else in coffee script

Question

Folks,

I have two text boxes in my html. I wanted to compare their values using coffee script. Although I have googled about it, and I am kind of sure that I am doing what is expected to do but still I see a strange behavior.

Thing is :

Lets say, I have two text boxes with id as "title" and "author". Along with that, I have a button which onclick triggers the coffee script function.

My coffee script function looks as:

check_fields = ->
  elem_book_title  = $("#title").val()
  elem_book_author = $("#author").val()
  alert(elem_book_title)
  alert(elem_book_author)
  if not elem_book_title? 
    alert("title is null")
  else if not elem_book_author?
    alert("author is null")
  else
    alert("both are ok")

Situation is, If I enter something only in my "title" textbox, it should alert me that "author is null". Right? But surprisingly, it alerts me that "both are ok".. Expected? Or I missed something?

what does it alert when you do `alert(elem_book_title.val())` ? — Mox Shah, Apr 06 '15 at 11:06
@Shomz, Well. Yes, I tried that too.. Something like: `elem_book_title = $("#title").val()` `elem_book_author = $("#author").val()` `alert(elem_book_title)` `alert(elem_book_author)` Rest code is same. But of no use. — Rahul Bhargava, Apr 06 '15 at 11:08
Sorry for editing the post many times. I was not getting the `code` right. — Rahul Bhargava, Apr 06 '15 at 11:10
jQuery constructor function _always_ returns an object - that is a _truthy_ value. You _should_ rewrite your code so that the values of those inputs are checked. — raina77ow, Apr 06 '15 at 11:14
@raina77ow.. Please see my above comment.. I have tried .val as well. — Rahul Bhargava, Apr 06 '15 at 11:15
Ahem. It's not 'as well'. It's the only proper way to do it. It doesn't guarantee that the rest of your code doesn't have an error, yes. But if you don't fix 'using the objects in checks' error, it's meaningless to advance further. — raina77ow, Apr 06 '15 at 11:17
@raina77ow.. Ahem. I fixed that and it still alerts the same "both are ok". What now? — Rahul Bhargava, Apr 06 '15 at 11:18
Could you show how your code looks like after the fix, editing the question accordingly? — raina77ow, Apr 06 '15 at 11:19
@raina77ow.. I have edited the code in question itself. Please have a look. — Rahul Bhargava, Apr 06 '15 at 11:24
_Now_ the answer given by @jcuenod fits your question precisely. And any other visitor of this page won't have to check the comments to understand what's wrong. That was my point actually. — raina77ow, Apr 06 '15 at 11:27

score 6 · Accepted Answer · edited May 23 '17 at 11:53

6

In jQuery, .val() won't return null for an empty field (except for a select element). Right now your coffee script evaluates to:

if (elem_book_title == null) {
  return alert("title is null");
} else if (elem_book_author == null) {
  return alert("author is null");
} else {
  return alert("both are ok");
}

So try removing the question marks

if not elem_book_title
  alert("title is null")
else if not elem_book_author
  alert("author is null")
else
  alert("both are ok")

This will produce the js I think you were expecting (which is testing for falsiness like an empty string, 0 or null):

if (!elem_book_title) {
  return alert("title is null");
} else if (!elem_book_author) {
  return alert("author is null");
} else {
  return alert("both are ok");
}

There is a question about the way coffeescript's existential operator (?) works here that you may find informative (thanks @raina77ow).

edited May 23 '17 at 11:53

Community

1
1

answered Apr 06 '15 at 11:14

jcuenod

55,835
14
65
102

1

It doesn't need null to be falsy. – Shomz Apr 06 '15 at 11:19
Am I really supposed to check the length explicitly? I thought there must be something to avoid that.. I thought. – Rahul Bhargava Apr 06 '15 at 11:20
But look at what your coffeescript evaluates to: `if (elem_book_title == null)`. If you remove the question marks then it will test falsiness... I've updated my answer. – jcuenod Apr 06 '15 at 11:21
1

You can point the OP to [this question](http://stackoverflow.com/questions/17253772/how-does-coffeescripts-existential-operator-work) which quotes the documentation on `?`, explaining why it's not needed here. – raina77ow Apr 06 '15 at 11:29

score 0 · Answer 2 · answered Apr 06 '15 at 11:24

you should either check for "" (blank), because when there's no value entered in textbox, .val() function will return "" .

you can do something like this:

check_fields = ->
  elem_book_title  = ""
  elem_book_author = "asdasd"

  if not elem_book_title? or  elem_book_title == "" 
    alert("title is null")
  else if not elem_book_author? or  elem_book_author == "" 
    alert("author is null")
  else
    alert("both are ok")

Generated JS

var check_fields;

check_fields = function() {
  var elem_book_author, elem_book_title;
  elem_book_title = "";
  elem_book_author = "asdasd";
  if ((elem_book_title == null) || elem_book_title === "") {
    return alert("title is null");
  } else if ((elem_book_author == null) || elem_book_author === "") {
    return alert("author is null");
  } else {
    return alert("both are ok");
  }
};

But an empty string is still falsey (as are `null` and `undefined`) in CoffeeScript so `!elem_book_title` sufficient. — mu is too short, Apr 06 '15 at 16:55

score 0 · Answer 3 · answered Apr 06 '15 at 17:39

tip: when debugging if-not-else-if-not-else I find it is easier to follow code using unless and returning in the block

return alert("title is null") unless elem_book_title
return alert("author is null") unless elem_book_author
alert("both are ok")

which produces similar JS

if (!elem_book_title) {
  return alert("title is null");
}
if (!elem_book_author) {
  return alert("author is null");
}
alert("both are ok");

if else in coffee script

3 Answers3