2026.4.1
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Test if undefined: undefined

The undefined test checks whether a variable has not been defined in the current template context. It returns true if the variable does not exist and false if it has been assigned a value. It is the opposite of defined.

This is useful when you want to explicitly detect missing variables and handle the absence case. For instance, you might want to set a default value or skip a section of the template when a particular variable has not been provided. While default is often more concise for fallback values, undefined gives you full control with {% if %} blocks.

Usage

Here’s how to use this template function. Copy any example and adjust it to your setup.

As a test 
{% if my_variable is undefined %}
  Variable is missing
{% endif %}
Result (stringA piece of text, like a name, message, or entity ID. In templates, wrap strings in quotes, like "living_room" or "lights are on".) 
Variable is missing

Function signature

The signature is a technical summary of this template function. It shows the name of the function, the values (called parameters) it accepts, and what type of data each parameter expects (for example, a piece of text or a number).

Function parameters that have a = with a value after them are optional. If you leave them out, the default value shown is used automatically. Function parameters without a default are required.

undefined(
    value: Any,
) -> bool

Function parameters

The following parameters can be provided to this test.

value any Required

The variable to test. Returns true if the variable is not defined in the current context.

Good to know

  • A variable explicitly set to none is still defined, so this test returns false for it. Use is none to check for that case.
  • Useful for template macros or scripts where callers may or may not pass an argument.

Try it yourself

Ready to test this? Open Developer tools > Template, paste the example into the Template editor, and watch the result update on the right. Edit the values to see how the function adapts to your own entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this function comes up in automations and templates. Copy any example and adapt it to your setup.

Provide fallback logic for missing data

When an optional variable is not provided, supply alternative content.

TemplateA template is an automation definition that can include variables for the action or data from the trigger values. This allows automations to generate dynamic actions. [Learn more] 
{% if custom_greeting is undefined %}
  Welcome home!
{% else %}
  {{ custom_greeting }}
{% endif %}
Result (stringA piece of text, like a name, message, or entity ID. In templates, wrap strings in quotes, like "living_room" or "lights are on".) 
Welcome home!

Skip optional sections

Only render a section when the required data is available.

TemplateA template is an automation definition that can include variables for the action or data from the trigger values. This allows automations to generate dynamic actions. [Learn more] 
Status report:
{% if details is undefined %}
  No additional details available.
{% else %}
  {{ details }}
{% endif %}
Result (stringA piece of text, like a name, message, or entity ID. In templates, wrap strings in quotes, like "living_room" or "lights are on".) 
Status report:
  No additional details available.

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with your template and expected result, or share on our subreddit /r/homeassistant.

Tip

AI assistants like ChatGPT or Claude can also explain or fix templates when you describe what you want in plain language.

Related template functions

These functions work well alongside this one: