Incorrect use of assign or capture

Liquid, the templating language used in platforms like Shopify, Jekyll, and Power Pages, provides two key tags for working with variables: assign and capture. Both of these tags are used to store data, but they have different purposes and scopes of use. Misusing them can lead to errors, incorrect variable assignments, or unexpected behavior in your templates.

Let’s break down what assign and capture do, common mistakes when using them, and how to fix them.

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1. Understanding assign and capture in Liquid

• assign: This is used to assign a value to a variable. Once a variable is assigned, it can be used throughout the template in that context. Syntax: liquidCopyEdit{% assign variable_name = value %}
• capture: This is similar to assign, but it is specifically used for capturing a block of text or content inside a variable. It’s useful when you need to store large blocks of HTML or strings. Syntax: liquidCopyEdit{% capture variable_name %} <!-- content here --> {% endcapture %}

The main difference between them is that assign is for simple value assignments (like strings or numbers), while capture is for content that might span multiple lines.

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2. Common Mistakes in Using assign and capture

Mistake 1: Assigning Non-Existent or Undefined Variables

When you try to assign or capture a variable that doesn’t exist or isn’t properly initialized, Liquid will not throw an error but will result in an empty variable or undefined content.

Example:

{% assign product_price = product.price %}

If product or product.price is undefined or null, product_price will be empty.

Fix: Ensure the variable or object you are referring to exists. You can add a check using if to ensure that the value exists before assignment.

{% if product %}
  {% assign product_price = product.price %}
{% else %}
  {% assign product_price = 0 %}
{% endif %}

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Mistake 2: Incorrect Use of capture with assign

Sometimes, users mistakenly combine capture and assign in a way that doesn’t make sense, such as using capture with assign in one step.

Example of Incorrect Use:

{% assign greeting = {% capture "Hello, world!" %} %}

Fix: capture doesn’t return values directly to be used in assign. Instead, capture should be used to store a block of content into a variable, and you can use assign to store simple variables.

Correct usage:

{% capture greeting %}
  Hello, world!
{% endcapture %}

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Mistake 3: Trying to Assign or Capture Multiple Variables in One Line

It’s common to see people try to assign multiple variables at once in a single line, which is not valid in Liquid.

Example:

{% assign first_name, last_name = "John", "Doe" %}

This will throw an error because Liquid doesn’t support multiple assignments in one line.

Fix: Assign each variable individually on separate lines.

{% assign first_name = "John" %}
{% assign last_name = "Doe" %}

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Mistake 4: Using capture for Simple Values

While capture is powerful for storing large blocks of text or HTML, it’s not necessary for simple string values. Overusing capture for simple values can lead to unnecessary complexity in your templates.

Example of Incorrect Use:

{% capture greeting %}
  "Hello, world!"
{% endcapture %}

Fix: For simple strings, use assign instead.

{% assign greeting = "Hello, world!" %}

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Mistake 5: Forgetting to Close capture Block

When using capture, you need to ensure that the block is properly closed with {% endcapture %}. If you forget to close the block, Liquid will throw an error.

Example of Incorrect Usage:

{% capture greeting %}
  Hello, world!

Fix: Always ensure that you close the capture block properly.

{% capture greeting %}
  Hello, world!
{% endcapture %}

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Mistake 6: Using assign and capture in Incorrect Contexts

assign and capture are not always interchangeable. For instance, capture is designed to store blocks of text, while assign is for scalar values. Using them incorrectly can result in data loss or unexpected behavior.

Example:

{% capture greeting %}
  {% assign name = "John" %}
{% endcapture %}

This will not work as expected because capture is meant to store content, not the output of an assign tag.

Fix: Use assign separately to store variables and then use capture to store larger blocks of content.

{% assign name = "John" %}
{% capture greeting %}
  Hello, {{ name }}!
{% endcapture %}

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Mistake 7: Using capture with for Loops Improperly

Using capture inside for loops can also cause issues if the loop content isn’t properly enclosed or if you’re trying to capture too much data.

Example:

{% capture product_names %}
  {% for product in products %}
    {{ product.name }}
  {% endfor %}
{% endcapture %}

If the loop outputs a large amount of data, this may not behave as expected or could exceed limits for rendering.

Fix: Use capture for smaller blocks of text, or consider using assign for individual product names in loops.

{% for product in products %}
  {% assign product_name = product.name %}
{% endfor %}

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3. Debugging assign and capture Errors

When troubleshooting assign and capture errors, keep these steps in mind:

1. Check for Typos or Misspellings: Ensure that variable names are consistent throughout the code. Liquid is case-sensitive, so productName and ProductName would be treated as different variables.
2. Check for Proper Block Closures: Always ensure that any capture blocks are properly closed with {% endcapture %}.
3. Ensure Variables Are Initialized: Before using a variable, confirm that it exists or is assigned a value. This prevents errors due to undefined variables.
4. Use Debugging Output: Use {{ variable_name }} to print out the values of variables to confirm they are being set correctly.
5. Check Data Type Compatibility: Ensure that you’re not using capture for simple variables and that you’re not overcomplicating your code with unnecessary use of capture.

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4. Summary of Best Practices

• Use assign for simple data types like strings, numbers, and objects.
• Use capture for larger blocks of HTML or content that span multiple lines or include dynamic content.
• Avoid using capture for simple scalar values—this is unnecessary and can add complexity.
• Always ensure variables are initialized before using them in templates to prevent undefined values.
• Avoid combining assign and capture inappropriately—use each tag for its intended purpose.