When working with Liquid in Power Pages or other platforms that support it, you may encounter an error or unexpected behavior when trying to render an undefined variable. This issue typically occurs when you attempt to output a variable that hasn’t been assigned a value or doesn’t exist in the context where you’re trying to access it.
Below is a detailed guide to help you troubleshoot and resolve the “Rendering Undefined Variable” error in Liquid.
Common Causes of Undefined Variable Error
- Variable Not Assigned
- Scope of the Variable
- Typographical Errors
- Improper Variable Passing in Context
- Empty or Nil Values
- Missing Data or API Response
- Incorrect Filter Usage
- Liquid Context Misunderstanding
1. Variable Not Assigned
One of the most common causes of the “undefined variable” error is that the variable you are trying to render has not been assigned a value. If Liquid can’t find the variable in the current scope, it will return an error.
Example of Undefined Variable:
<p>{{ user.name }}</p>
In this example, if user has not been assigned a value, Liquid will throw an error.
Fix: Make sure the variable is assigned properly before trying to use it. For instance:
{% assign user = contact %}
<p>{{ user.name }}</p>
2. Scope of the Variable
Variables in Liquid can be scoped to a specific context, such as within a loop or conditional block. If you try to access a variable outside of its scope, it may be undefined.
Example of Scope Issue:
{% assign user = contact %}
{% if user %}
<p>{{ user.name }}</p>
{% endif %}
<p>{{ user.name }}</p> <!-- This could be undefined outside the if block -->
Fix: Ensure that the variable is assigned or used within the correct scope:
{% assign user = contact %}
{% if user %}
<p>{{ user.name }}</p>
{% else %}
<p>No user data available</p>
{% endif %}
3. Typographical Errors
A common issue when rendering variables is simply a typo in the variable name. Liquid is case-sensitive, so even small typographical errors can lead to the “undefined variable” error.
Example of Typo:
{% assign product = "Laptop" %}
<p>{{ Prodct }}</p> <!-- Typo here -->
Fix: Always double-check the variable names for any typographical errors. Ensure that you are using the correct case and spelling:
{% assign product = "Laptop" %}
<p>{{ product }}</p> <!-- Correct variable name -->
4. Improper Variable Passing in Context
If you are passing variables from a higher context (such as a parent template) to a child template, and the variable is not correctly passed, it may result in an undefined variable error.
Example of Missing Context:
<!-- Parent template -->
{% include 'child_template' %}
<!-- Child template -->
<p>{{ user.name }}</p> <!-- user variable not passed to the child template -->
Fix: Ensure that you explicitly pass variables when including templates:
<!-- Parent template -->
{% assign user = contact %}
{% include 'child_template' with user %}
<!-- Child template -->
<p>{{ user.name }}</p> <!-- Now 'user' is passed properly -->
5. Empty or Nil Values
In Liquid, nil or empty values are considered falsy. If a variable is not assigned any value or is explicitly set to nil, trying to render it may result in an undefined variable error.
Example of Nil Variable:
{% assign user = nil %}
<p>{{ user.name }}</p> <!-- user is nil -->
Fix: Before rendering the variable, check whether it is nil or empty to avoid errors:
{% if user != nil %}
<p>{{ user.name }}</p>
{% else %}
<p>No user data available</p>
{% endif %}
6. Missing Data or API Response
In cases where you’re fetching data dynamically (e.g., from an API or a database), an undefined variable can occur if the data source does not return the expected response or if the API call fails.
Example of Missing Data:
{% assign product = api_call("product_id") %}
<p>{{ product.name }}</p> <!-- product might be nil if the API call failed -->
Fix: Make sure the API call or data fetch operation is successful and that it returns the expected data. Additionally, check for errors before rendering:
{% assign product = api_call("product_id") %}
{% if product != nil %}
<p>{{ product.name }}</p>
{% else %}
<p>Product not found</p>
{% endif %}
7. Incorrect Filter Usage
Sometimes, the use of Liquid filters can result in unexpected behavior, especially when applied to undefined variables. If a filter is applied to a variable that is nil or undefined, it can cause errors.
Example of Incorrect Filter Usage:
{% assign product_name = nil %}
<p>{{ product_name | upcase }}</p> <!-- product_name is nil -->
Fix: Make sure that you are applying filters to variables that are defined, or use conditionals to prevent applying filters to undefined or nil values:
{% if product_name %}
<p>{{ product_name | upcase }}</p>
{% else %}
<p>No product name available</p>
{% endif %}
8. Liquid Context Misunderstanding
Understanding how Liquid variables are passed in different contexts is crucial. If you’re using Liquid within a CMS or a platform like Power Pages, ensure that the data is available in the context you’re using it. Sometimes, variables are accessible in one context but not in another.
Example of Context Misunderstanding:
{% assign user = contact %}
{% render 'profile_page' %} <!-- profile_page might not have access to 'user' -->
Fix: Ensure the variable is passed correctly between contexts:
{% assign user = contact %}
{% render 'profile_page' with user %}
Summary Table of Fixes
| Issue | Cause | Fix |
|---|---|---|
| Variable Not Assigned | Variable not defined | Assign the variable correctly before use |
| Scope of the Variable | Variable is out of scope | Ensure the variable is available in the scope |
| Typographical Errors | Typo in the variable name | Check for spelling/case sensitivity errors |
| Improper Variable Passing | Variable not passed to template | Explicitly pass variables to included templates |
| Empty or Nil Values | Variable is nil or empty | Check for nil or empty values before rendering |
| Missing Data or API Response | Data source or API call fails | Ensure successful data fetching and error handling |
| Incorrect Filter Usage | Filter applied to undefined variable | Apply filters only to defined variables |
| Liquid Context Misunderstanding | Variable scope misunderstood | Ensure proper context passing for variables |
Example Code for Handling Undefined Variables
{% assign user = contact %}
{% if user %}
<p>{{ user.name }}</p>
{% else %}
<p>User not found</p>
{% endif %}
In this code, user is checked for existence before trying to render its property, ensuring that an undefined variable is not rendered.