How do I check if a variable is defined?

[casey-chow]

Say (in Jekyll, for example) I want to check if a certain variable, say header_image was defined and do something if it is, and something else if it isn't. How would I do that?

[tobi]

why not check against nil. Nil and "doesn't exist" are the same concept in liquid (unlike JS).

{% if header_image %} .... {% endif %}

[casey-chow]

Ah. Thanks.

[dvdsgl]

What if the possibly undefined variable is a boolean, and you want to run the block if it's true or false, but not if it's undefined?

[fw42]

then you can use {% if header_image == true or header_image == false %}

[bristoljon]

What about negating this check?

In my case the variable row.image may or may not be defined. If it is I want to show an <img> tag but hide a different background <div>.

Although {% if row.image %} works as expected, the only way I've found to negate it is:

{% if row.image %}
{% else %}
  <div>...</div>
{% endif %}

Is there a more succinct form e.g. {% if !row.image %}?

[fw42]

There is an unless tag

[dyspop]

the unless tag is the inverse of the if tag and sounds like it will work for you

[bristoljon]

Thanks @fw42 and @dyspop, that's exactly what I was looking for and it was in the docs right below if!

[anetacamo]

But if I want to check for a condition OR if the variable is not defined at all
lets say something like this

{% if row.number == 0 or !column.number  %}
something is missing
{% else %}
for loop
{% endif %}

I would prefer to write it in as few statements as possible because the code else if is large.
Is that possible? Thanks :)

[steinbachr]

For anyone who stumbles here and - like me - was looking for this check in the context of a Jekyll list filter (e.g. where_exp), I was able to eventually get it with where_exp: "item", "item.header_image != nil".

Unfortunately, it's pretty unclear from the language documentation what the difference between empty, nil, and blank is, so required some trial and error to get working.

[Ynjxsjmh]

{% if row.number == 0 or !column.number %}
something is missing
{% else %}
for loop
{% endif %}

I do this by

{% if page.type == "asciidoc" %}
  {% include article/asciidoc.html %}
{% elsif page.type == "readtheorg" %}
  {% include article/readtheorg.html %}
{% elsif page.type == "markdown" or page.type == "" or page.type == nil %}
  {% include article/markdown.html %}
{% else %}
  "Post type invalid, your type is {{page.type}}, which is not a valid type."
{% endif %}

I think page.type == "" can check if type varaible is defined in page. If true, type is not defined.