The | default filter returns the value passed as the first argument if the filtered value is undefined or empty, otherwise the filtered value is returned.

{{ variable | default('The variable is not defined') }}

{{ | default('The foo property on variable is not defined') }}

{{ variable['foo'] | default('The foo key in variable is not defined') }}

{{ '' | default('The variable is empty')  }}

When using the default filter on an expression that uses variables in some method calls, be sure to use the default filter whenever a variable can be undefined:

{{ variable.method(foo | default('bar')) | default('bar') }}

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