How to strip leading whitespace before a Nunjucks variable only if the variable string is empty? - nunjucks

Question

I'd like to support construction of Nunjucks templates where leading whitespace before variables is only stripped if the variable is empty.
A simple example could begin:
Hello {{ firstname }}, how are you?
If firstname is "Dave", this renders to "Hello Dave, how are you?". But if not set, it renders to "Hello , how are you?" with the extra space.
How can templates be constructed, in a general way (without fore-knowledge of the surrounding text), such that leading whitespace is only stripped before variables if the variable string is empty?
Using {{- firstname }} would always strip leading whitespace (so doesn't work if the variable is not empty, resulting in "HelloDave" in the example above
).
Something like the following works for this specific case (or better, the approaches suggested by #AikonMogwai in the comments), but is overly complex and relies on knowledge of the surrounding text (e.g., inserts a space if the variable is non-empty):
Hello
{%- if firstname %}
{{- " " + firstname -}}
{% else %}
{{- "" -}}
{% endif %},
Here's a fiddle that demonstrates the above examples: https://jsfiddle.net/davebeyer/3L5146jg/ (Click "Run" then "Render".)
Ideally, I'd like to create/use a custom tag (like {{=) or custom filter (like condlstrip for conditional left-strip) so that templates can be constructed using something like:
{{= firstname }}
or
{{ firstname | condlstrip }}
But it's not clear to me how to create a custom {{= tag, and also not clear whether filters, like condlstrip, can affect rendering outside of the tags.
Regarding the requirement "without fore-knowledge of the surrounding text," this specifically means that the solution should not assume that there will always be a space in front of the variable. For instance, it should also work in a case like the following (using my wished-for {{= tag), and where the user is fairly confident that school and studentId are set, but firstname may or may not be set):
Hi {{= firstname }}, your user ID is {{= school }}-{{= studentId }} and you can reach your profile at http://{{= school }}.example.com/profiles/{{= studentId }}
to produce:
Hi Dave, your user ID is CentralValley-1234 and you can reach your profile at http://CentralValley.example.com/profiles/1234
I.e., it would be great if our system could use the pattern {{= variable_name }} to produce reasonable default behavior for inserted variables in templates created by users (who are actually using a graphical, higher-level UI).

Answer 1

A simpler solution:
<li>Hello{{ " " if user.firstname }}{{ user.firstname }}, how are you?</li>
https://jsfiddle.net/pn8koeg3/

Answer 2

var nunjucks = require('nunjucks');
var env = nunjucks.configure();
env.addFilter('ws', val => !val || val == 0 ? val : ' ' + val);
var html = env.renderString(`
Hi {{- firstname | ws -}}, your user ID ...
`, { firstname: 'Dave' });
console.log(html);