What's the difference between "is defined" and "is not null"?

Question

{% if var is defined %}

and

{% if var is not null %}

Is there a difference in usage of them?

Answer 1

is defined - The variable has been defined, and set to any value (or null).

is not null - The variable has been defined, and is specifically not null.

In PHP terms, it's like the difference between is_null() vs. checking whether the variable exists using get_defined_vars() (see this Stack Overflow thread for a little more clarity)

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Try this experiment... Without setting myVar, put this in your template:

{{ (myVar is defined) }}

Since that equates to false, nothing will appear in your template. Now change it to this:

{{ (myVar is not null) }}

That will actually throw an error, since myVar was never defined.

Answer 2

The first statement checks for the existence of var, the second checks that it has a value other than null and will throw and error if var does not exist.

If you're not sure whether a variable exists, you should check if it exists first then check for its value to be safe.

You could combine both statements like so {% if var is defined and var is not null %} or use empty, not empty if you're feeling fancy: )