Strict mode · Language

Strict mode is on by default. It turns a misspelt variable or a missing property into an error, instead of a blank space on the page that nobody notices.

page.kiln

<h1>{{ titel }}</h1>

Error

Unknown variable "titel" in page.kiln at line 1, column 8. Did you mean "title"?

Optional data

When data may be missing, say so in the template. These never raise missing-variable or missing-property errors:

?. returns null if the left side is null: page?.author?.name
?? uses a fallback: page.subtitle ?? ""
default uses a fallback when the value is empty: page.subtitle|default("")
is defined checks whether a value exists: {% if page.subtitle is defined %}

page.kiln

<h1>{{ page.title }}</h1>
<p>{{ page.subtitle ?? "No subtitle" }}</p>

Output

<h1>About</h1>
<p>No subtitle</p>

Strict and lenient

Situation	Strict	Lenient
Unknown variable	Error	null
Missing property	Error	null
Property of null	Error	null
Required prop missing	Error	null
Prop not declared	Error	Ignored
Prop of the wrong type	Error	Not checked
Unknown filter, function, test or tag	Error	Error
Type errors, such as `"a" + 1`	Error	Error

Turning it off

Lenient mode treats missing data as null. Turn it on for the whole environment:

const kiln = createKiln({ loader, strict: false });

We recommend leaving strict mode on. Guarding optional data takes a few characters and catches real bugs.